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SOVAFA ACA Sociedad Venezolana de Aficionados a la Astronomía Asociación Carabobeña de Astronomía Mensajero Estelar Año 39 Nº 72 Octubre- Diciembre de 2014 Contenido: - Noticias - ¿Llego el Voyager a la Heliopausa? Radiantes del Trimestre - La Supernova más brillante de la historia Fases de la Luna - Infrared Photometry of the Pleyades Cúmulo Estelar de las Híades - Nuevo Ciclo del Calendario Maya Eclipse Total de Luna de Oct. 08 - ¿Por qué la Luna no es una esfera perfecta? ¿Cuánto de la superficie lunar vemos? - Methane Plumes in the Arctic Solar Variability and Terrestral Climate - Olas de 5 m de altura erosionan el Hielo Ártico ¿Pudo el Bosón de Higgs Colapsar el U. - Old pre-main-sequence stars Ocultación de Marte por la Luna - Temperaturas Anómalas en Julio y Agosto La GMR de Júpiter se achica - Deflexión de la luz por el Sol Nuevas Enanas Rojas cercanas al Sol - Born Betwen Nov. 29 and Dec. 18 … A new wiew of the red planet - Planetas Acuosos Violenta historia del Sol joven… - Meteorito en Nicaragua Junio de 2014, el más cálido registrado - Técnica Lucky Image… - Las Geminíadas www.sovafa.com, www.sovafa.org, [email protected], @astrorecord, @sovafa Noticias 1.- La Corona Solar es mucho más grande que lo que se pensaba. Recientes observaciones realizadas con el satélite STEREO evidencian que esta se extiende más de 8 millones de km de la superficie del Sol. 2.- Datos obtenidos por la sonda Cassini parecen indicar que el interior de Titán podría contener un océano muy salado, de acuerdo a mediciones de micro gravedad realizadas por la sonda. Estas sales serían de Azufre, Sodio, y Potasio. 3.- El Observatorio “Athena” fue incorporado a la visión cósmica de la Agencia Espacial Europea, ESA, en su plan 2015 – 2025. El mismo estudiará el Universo caliente y energético, estará en el punto de equilibrio gravitatorio “Lagrange 2 (L2), previéndose su lanzamiento para el año 2028. 4.- Un equipo de investigadores de la Universidad de Nueva Gales del Sur, en Australia, ha descubierto un planeta similar a la Tierra potencialmente habitable a tan sólo 16 años luz de distancia. Llamado Gliese 832, es una "súper-Tierra" con una masa 5 veces la de nuestro planeta. Tarda 16 días en completar una órbita alrededor de su estrella; una enana roja cuyo brillo es menor al del Sol, pero debido al tiempo que tarda en orbitarla tiene aproximadamente la misma energía estelar que la Tierra. 5.- En Julio la actividad solar volvió a disminuir muy drásticamente. Luego del Máximo a que llegó el año pasado, esta actividad decayó de manera bastante brusca, para luego, desde abril pasado volver a incrementarse, y en Julio volvió a caer de manera brusca. 6.- El asteroide 2014 KM4 de 192 metros descubierto, a principios mayo, ha pasado de forma segura por el sistema Tierra-Luna a 0.17 AU de distancia el 21 de abril. Hasta el momento, la trayectoria lo lleva a una ruta de colisión con el gigante del Sistema Solar, Júpiter en el año 2022. 7.- El Exoplaneta OGLE-2013-BLG-0341LBb, situado a 3.000 A.L. de la Tierra, posee condiciones muy parecidas a las del entorno terrestre. Es más frío que la Tierra, pues su estrella es más pequeña que el Sol, pero es un objeto potencialmente habitable. 8.- El 2 de Septiembre se descubrió un asteroide que nos pasó a unos 38.000 km dos días antes. El objeto mide solo unos 8 m de diámetro y fue catalogado como 2014 RA 9.- El día 3 de Septiembre se observaron 305 bolas de fuego sobre el SE de USA, el número más elevado que se ha observado hasta ahora. Este año solo el día 13 de Agosto, máximo de las Perseidas se detectaron unos 105 bólidos, y la gran mayoría de ellos conectados con el radiante. 10.- En Agosto la periodista y locutora Amalia Heller entrevistó a Jesús Otero por Mágica 99.1 FM sobre la lluvia de estrellas de las Perseidas y la Conjunción de Venus y Júpiter, en su programa La Magia de Amalia Heller que se transmite de Lunes a Viernes de 7:00 a 8:30 pm 11.- El 04 de Septiembre Amalia Heller nuevamente entrevistó a Jesús Otero, pero esta vez sobre el Asteroide 2014 RC que nos pasó rozando el día 07 de septiembre. 12.- El 31 de Agosto y el 07 de Septiembre 2 asteroides pasaron a unos 40.000 Km de la Tierra, esto es unos 33.000 km de la Superficie terrestre, sabemos que estos pasos rasantes son comunes, pero también sabemos que en algún momento seremos impactados. 13.- Un instrumento de la NASA abordo del orbitador Rosetta de la ESA, ALICE, descubrió que el cometa 67P/Churyumov-Gerasimenko es inusualmente oscuro, muy oscuro. Analizando las ondas ultravioletas de la superficie del cometa, también detectó la presencia de hidrogeno y oxigeno en su coma y pocas evidencias de hielo de agua expuesto. 14.- El 6 de septiembre cayó un meteorito en Nicaragua. Hubo un fuerte destello en el cielo, luego una fuerte explosión, e instantes después un sismo suave producido por el impacto. El objeto dejó un cráter de 12 m de diámetro y unos 5 de profundidad, el objeto pudo haber tenido casi un metro de diámetro y debe encontrarse bajo los rellenos post impacto del cráter. 15.- El día 06 de Septiembre se observó un bólido de magnitud -12 con ruido sobre la ciudad de Barcelona, España. En Agosto y Septiembre la Tierra estuvo pasando por zonas muy densas de restos interplanetarios. De hecho el 03 de Septiembre se contabilizaron 305 Bolas de Fuego, solo en el SE de USA, y el 06 hubo 68. 16.- El 01 de Sepiembre, en Campinas, Brazil 4 brillantes bolas de fuego cruzaron el cielo con magnitude de -4 a -8, Agosto y Septiembre registraron muchos meteoros brillantes y la caída de un objeto al Norte de Managua, Nicaragua. 17.- Estudios recientes del USGS demuestran que es alktamente improbable que el volcán de Yosemite haga erpción en el futuro cercano. 18.- Evidencia de tectónica de placas en la Luna Europa de Júpiter. Se observó lo que parece una tectónica de Subducción en las capas de hielo del satélite. 19.- La Vía Láctea forma parte de un supercúmulo que ha sido bautizado como Supercúmulo de Laniakea, o cielo inmenso, que tiene 500 millones de Años Luz de Grosor. Esto fue descubierto con el Green Bank Radiotelescope. Este filamento contiene al menos 100.000 galaxias. Radiantes del Trimestre Radiante Oriónidas Taúridas del Sur Taúridas del N. Leónidas Androménidas 5185C.Minóridas 51 Androménidas Piscidas 43 Taúridas Geminíadas Púpidas - Vélidas Fecha Octubre 17 - 26 Sept.15-Nov 30 Sept. 19-Dic. 5 Nov. 14 - 20 Nov. 4 - 20 Dic. 1 - 5 Diciembre 04 Dic. 10 - 14 Dic. ¿? - 13 Dic. 13 - 16 Nov. 24 - Ene 9 Máximo Oct. 19 - 23 Nov. 3 Nov. 13 Nov. 17 - 18 Nov. 16 Dic. 3 - 4 Dic. 04 Dic. 10 Dic. 11 Dic. 12 - 13 Dic. 25 T. H. Z. 20 7 9 Var. Var. Var. ¿40? +80 ¿97? 145 15 A. R. 06h 18m 03h 22m 03h 53m 10h 12m 01h 44m 07h 36m + 15º + 13.6º + 22º + 22º + 25º + 4º 04h 10m 07h 28m 09h 03m +19.5º + 33º - 48º Hora 02:00 23:00 23:00 02:00 21:00 22:00 19:30 19:00 20:00 22:30 00:00 Las Leónidas, las 43 Taúridas, y las Geminíadas son radiantes que dan meteoros brillantes y su número puede variar mucho de un año a otro. Las lluvias de estrellas aquí listadas se encuentran todas activas, algunas de ellas son de difícil observación pues sus meteoros son de poco brillo. Hay que ver cuál es la fase lunar el día de la observación, pues la luz de la Luna puede afectar mucho la observación del radiante. Máximo es el día en que se espera que la lluvia de estrellas llegue a su máximo número de meteoros. THZ es el número de meteoros que veríamos del radiante si este se encontrara en el zenit. α y δ son Ascensión Recta y Declinación. Hora se refiere a la hora en la cual puede empezar a observarse el radiante. Viene en Hora Legal de Venezuela. O -4,5h GMT. Este año las Oriónidas ocurrirán entre Cuarto Menguante y Luna Llena. Geminíadas y 43 Taúridas no serán molestadas por la Luna. Las Taúridas del Sur serán molestadas por Luna casi Llena, al igual que 51 Androménidas. Las Púpidas - Vélidas ocurrirán en Luna Nueva y la Luna no interferirá en su observación Si observa cualquiera de estos radiantes o una actividad meteórica inusual envíe un informe a [email protected] o un mensaje al Twitter: αastrorecord Fases de la Luna Luna Nueva Fecha Hora Sept. 24 06:12 Oct. 23 21:55 P Nov. 22 12:31 Dic. 22 01:35 Cuarto Creciente Fecha Hora Oct. 01 19:32 Oct. 31 02:48 Nov. 29 10:06 Dic. 28 18:32 Luna Llena Fecha Hora Oct. 08 10:49 t Nov. 06 22:22 Dic. 06 12:26 Ene. 05 04:53 Cuarto Menguante Fecha Hora Oct.15 19:12 Nov. 14 15:17 Dic.14 12:53 Ene. 13 09:48 En Luna Nueva la Luna no se puede ver, pues está en Conjunción con el Sol. En Cuarto Creciente la Luna se observa en la Tarde y primeras horas de la noche. En Luna Llena la Luna sale al ocultarse el Sol y se observa durante toda la noche. En Cuarto Menguante la Luna sale tarde, se observa de madrugada y primeras horas de la mañana. Estos datos son muy importantes a la hora de planificar sus observaciones, ya sean planetarias, de radiantes u objetos de espacio profundo. Téngalas en cuenta para la observación de eventos astronómicos. t = Eclipse Total de Luna y A = Eclipse Anular de Sol El Eclipse Total de Luna de Octubre 08 podrá observarse alto en el firmamento en el momento de la totalidad. Este es un proyecto importante de observación y estamos involucrados en un proyecto internacional. P Significa Eclipse Parcial de Sol. No será visible en Venezuela. Cúmulo Estelar de las Híades Con la excepción del cúmulo de la Osa Mayor, las Híades es el cúmulo estelar más cercano a la Tierra, se encuentra a una distancia de 151 Años Luz (AL). Es muy fácil de identificar en el firmamento por ser un cúmulo compacto y con una forma de V muy característica, lo que lo hace un Asterismo. El cúmulo se ve alto en nuestro firmamento, entre las Pléyades y la Constelación de Orión. Utilizando la enfilación de las estrellas del Cinturón de Orión en sentido Alnitak – Mintaka y prolongando esta enfilación hacia el NW llegamos a él. La estrella más brillante que vemos en esta V de estrellas es la gigante roja Aldebarán, cuyo nombre significa el Ojo del Toro, y que en realidad no forma parte de él. Esta estrella cierra la V al Sur. De este cúmulo podemos observar a simple vista poco más de una docena de estrellas, pero varias decenas de ellas pueden observarse con pequeños binoculares. Las Híades eran ninfas, según la mitología griega e hijas de Atlas y Aethra, las cuales lloraban eternamente a su hermano Hyas, quien fue muerto por un León. Las Híades eran medio hermanas de las Pléyades, hijas de Atlas con Pleione. Los dioses colocaron a las Híades y a las Pléyades en el firmamento a propósito, para salvarlas de los deseos lujuriosos de Orión. Al mismo tiempo convirtió a Hyas en la constelación de Acuario, y al León que lo mató en la Constelación de Leo, en la parte opuesta del firmamento. Así cuando una constelación salía al Este, la otra se ocultaba al Oeste. Este mito muestra una ambivalencia con el mito de Orión y el Escorpión Celeste enviado por la Diosa Diana para que picara y diera muerte a Orión. Zeus da la inmortalidad a su hijo Orión convirtiéndolo en Constelación y ubica al Escorpión en el lado opuesto del cielo a fin de evitar un nuevo encuentro. Así cuando Orión sale al Este, Scorpio desaparece por el Oeste y viceversa. De esta manera nunca podemos ver ambas constelaciones en el firmamento al mismo tiempo. Con un telescopio se observan cerca de 100 estrellas de este cúmulo estelar. Si bien en la Mitología las Híades y las Pléyades son hermanas, en la realidad ambos cúmulos son muy diferentes. Las Pléyades poseen estrellas Azules muy jóvenes y su edad no supera los 100 millones de años, por su parte, Las Híades poseen muchas estrellas rojas, gigantes rojas y naranjas, así como enanas blancas, por lo que la edad de este cúmulo estelar es de más de 700 millones de años. Pero hay un cúmulo estelar abierto cuyas características son muy similares al de las Híades, es el Cúmulo del Pesebre, en Cáncer. Ambos cúmulos se mueven en dirección idéntica en el espacio y sus edades son similares. Algunos astrónomos creen que a pesar de encontrarse muy lejos uno del otro, ambos se formaron en la misma nebulosa hace unos 700 u 800 millones de años. Eclipse Total de Luna, Oct. 08, 2014 El 08 de Octubre de 2014 ocurrirá un Eclipse Total de Luna que será visible en Venezuela, desdichadamente la fase de totalidad no será visible desde Venezuela, pues la Luna se ocultará poco antes de la llegada de la Totalidad. Sin embargo podremos observar como la Luna se irá eclipsando mientras baja en el horizonte. Tiempos para Venezuela El Eclipse Penumbral Empieza: El Eclipse Parcial Empieza: El Eclipse Total Empieza: El Medio del Eclipse: El Eclipse Total Finaliza: El Eclipse Parcial Finaliza: El Eclipse Penumbral Termina: 08:15:33 UT 09:14:48 UT 10:25:10 UT 10:54:36 UT 11:24:00 UT 12:34:21 UT 13:33:43 UT 03:45:33 HLV 04:44:48 HLV 05:55:10 HLV 06:24:36 HLV 06:54:00 HLV: 08:04:21 HLV 09:03:43 HLV Los próximos Eclipses Lunares observables en Venezuela ocurrirán en Abril 04 de 2015, pero solo podrá el Inicio del Eclipse y eso con suerte, pues a Luna estará muy cerca del Horizonte. El siguiente será el 28 de Septiembre, este si será visible en su totalidad desde Venezuela. El Eclipse de Octubre 08 nos sirve para practicar la observación de este fenómeno, el paso de la sombra sobre cráteres, y mares, y realizar observaciones. El de Abril 04 no vale la pena observarlo, porque si llegáramos a ver algo sería el primer contacto, pero la Luna estará ya ocultándose. Suerte y recuerden enviarme sus observaciones a: [email protected] ¿Cuánto de la Luna podemos ver desde la Tierra? Jesús H. Otero A. En un momento determinado, nunca podemos ver más de un 50% de la superficie lunar, pero debido al movimiento de Libración, a lo largo del tiempo podemos observar hasta un 59% de la superficie de nuestro compañero planetario. Este movimiento que hace cabecear a nuestro satélite hacia el Norte y el Sur, y hacia los lados Este y Oeste, nos permite ver un 9% más de la superficie lunar. Desde casi su formación, cuando la Luna estaba unas 10 veces más cerca de la Tierra que ahora, el movimiento de rotación de la Luna fue frenado por la fuerza de las mareas gravitatorias, esto causo que se estableciera una rotación resonante 1:1, también llamada sincrónica, esto es, el astro de la noche tarda lo mismo en efectuar un giro sobre sí mismo, que en girar una vez alrededor de nuestro planeta, enseñándonos por ello siempre la misma cara. Habiendo así una cara visible y una cara oculta. Algunos dicen la cara oscura de la Luna, pero esto es un error, no hay una cara oscura, ambos lados reciben por igual luz solar. Si observamos la Luna por un tiempo veremos como parecen moverse los relieves lunares, notándose estos un poco más al Norte o Sur, o Este u Oeste. Esto puede notarse fácilmente en la foto arriba, donde el cráter Tycho pareciera estar más al Norte en la imagen de la derecha. Pero la Luna no solo cambia un poquito al Este – Oeste, y Norte – Sur, pues hay varios tipos de libraciones que hacen a este movimiento más complejo. Por si esto fuera poco la luna no exhibe siempre el mismo tamaño. Como la órbita lunar es elíptica, el tamaño aparente de la Luna también varía si se encuentra en Perihelio o Afelio, es decir en el momento más cercano o más lejano de su órbita. Solar Variability and Terrestrial Climate Dr. Tony Phillips, NASA Jan. 8, 2013: In the galactic scheme of things, the Sun is a remarkably constant star. While some stars exhibit dramatic pulsations, wildly yo-yoing in size and brightness, and sometimes even exploding, the luminosity of our own sun varies a measly 0.1% over the course of the 11-year solar cycle. There is, however, a dawning realization among researchers that even these apparently tiny variations can have a significant effect on terrestrial climate. A new report issued by the National Research Council (NRC), "The Effects of Solar Variability on Earth's Climate," lays out some of the surprisingly complex ways that solar activity can make itself felt on our planet. These six extreme UV images of the sun, taken by NASA's Solar Dynamics Observatory, track the rising level of solar activity as the sun ascends toward the peak of the latest 11-year sunspot cycle. Understanding the sun-climate connection requires a breadth of expertise in fields such as plasma physics, solar activity, atmospheric chemistry and fluid dynamics, energetic particle physics, and even terrestrial history. No single researcher has the full range of knowledge required to solve the problem. To make progress, the NRC had to assemble dozens of experts from many fields at a single workshop. The report summarizes their combined efforts to frame the problem in a truly multi-disciplinary context. One of the participants, Greg Kopp of the Laboratory for Atmospheric and Space Physics at the University of Colorado, pointed out that while the variations in luminosity over the 11-year solar cycle amount to only a tenth of a percent of the sun's total output, such a small fraction is still important. "Even typical short term variations of 0.1% in incident irradiance exceed all other energy sources (such as natural radioactivity in Earth's core) combined," he says. Of particular importance is the sun's extreme ultraviolet (EUV) radiation, which peaks during the years around solar maximum. Within the relatively narrow band of EUV wavelengths, the sun’s output varies not by a minuscule 0.1%, but by whopping factors of 10 or more. This can strongly affect the chemistry and thermal structure of the upper atmosphere. Space-borne measurements of the total solar irradiance (TSI) show ~0.1 percent variations with solar activity on 11-year and shorter timescales. These data have been corrected for calibration offsets between the various instruments used to measure TSI. SOURCE: Courtesy of Greg Kopp, University of Colorado. Several researchers discussed how changes in the upper atmosphere can trickle down to Earth Surface. There are many “Top Down” pathways for the Sun´s influence. For instance Charles Jackman of the Goddard Space Flight Center described how Nitrogen Oxides (NOx) created by solar energetic particles and cosmic rays in the stratosphere could reduce Ozone labels by a few percent. Because Ozone absorbs UV radiation, less Ozone means that more UV rays from the Sun would reach Earth surface. Several researchers discussed how changes in the upper atmosphere can trickle down to Earth Surface. Isaac Held of NOAA took this one step further. He describes how loss of Ozone in the stratosphere could alter the dynamics of the atmosphere below it. “the cooling of polar stratosphere associated with loss of Ozone increases the horizontal temperature gradient near the Tropopause”, he explains. “This alter the flux of angular momentum by mid-latitudes eddies. [Angular momentum is important because] the Angular momentum budget of troposphere controls the surface westerlies”. In other words, solar activity feld in the upper atmosphere can, through a complicate series of influences, push surface storm tracks off course. How incoming galactic cosmic rays and solar protons penetrate the atmosphere. SOURCE: C. Jackman, NASA Goddard Space Flight Center, “The Impact of Energetic Particle Precipitation on the Atmosphere,” presentation to the Workshop on the Effects of Solar Variability on Earth’s Climate, September 9, 2011. Many of the mechanisms proposed at the workshop had a Rube Goldberg-like quality. They relied on multi-step interactions between multiple layers of atmosphere and ocean, some relying on chemistry to get their work done, others leaning on thermodynamics or fluid physics. But just because something is complicated doesn't mean it's not real. Indeed, Gerald Meehl of the National Center for Atmospheric Research (NCAR) presented persuasive evidence that solar variability is leaving an imprint on climate, especially in the Pacific. According to the report, when researchers look at sea surface temperature data during sunspot peak years, the tropical Pacific shows a pronounced La Nina-like pattern, with a cooling of almost 1o C in the equatorial eastern Pacific. In addition, "there are signs of enhanced precipitation in the Pacific ITCZ (InterTropical Convergence Zone ) and SPCZ (South Pacific Convergence Zone) as well as abovenormal sea-level pressure in the midlatitude North and South Pacific," correlated with peaks in the sunspot cycle. The solar cycle signals are so strong in the Pacific, that Meehl and colleagues have begun to wonder if something in the Pacific climate system is acting to amplify them. "One of the mysteries regarding Earth's climate system ... is how the relatively small fluctuations of the 11year solar cycle can produce the magnitude of the observed climate signals in the tropical Pacific." Using supercomputer models of climate, they show that not only "top-down" but also "bottom-up" mechanisms involving atmosphere-ocean interactions are required to amplify solar forcing at the surface of the Pacific. Composite averages for December-January-February for peak solar years. SOURCE: G.A. Meehl, J.M. Arblaster, K. Matthes, F. Sassi, and H. van Loon, Amplifying the Pacific climate system response to a small 11 year solar cycle forcing, Science 325:1114-1118, 2009; reprinted with permission from AAAS. In recent years, researchers have considered the possibility that the sun plays a role in global warming. After all, the sun is the main source of heat for our planet. The NRC report suggests, however, that the influence of solar variability is more regional than global. The Pacific region is only one example. Caspar Amman of NCAR noted in the report that "When Earth's radiative balance is altered, as in the case of a change in solar cycle forcing, not all locations are affected equally. The equatorial central Pacific is generally cooler, the runoff from rivers in Peru is reduced, and drier conditions affect the western USA." Raymond Bradley of UMass, who has studied historical records of solar activity imprinted by radioisotopes in tree rings and ice cores, says that regional rainfall seems to be more affected than temperature. "If there is indeed a solar effect on climate, it is manifested by changes in general circulation rather than in a direct temperature signal." This fits in with the conclusion of the IPCC and previous NRC reports that solar variability is NOT the cause of global warming over the last 50 years. Much has been made of the probable connection between the Maunder Minimum, a 70-year deficit of sunspots in the late 17th-early 18th century, and the coldest part of the Little Ice Age, during which Europe and North America were subjected to bitterly cold winters. The mechanism for that regional cooling could have been a drop in the sun’s EUV output; this is, however, speculative. The yearly averaged sunspot number for a period of 400 years (1610-2010). SOURCE: Courtesy of NASA Marshall Space Flight Center. Dan Lubin of the Scripps Institution of Oceanography pointed out the value of looking at sun-like stars elsewhere in the Milky Way to determine the frequency of similar grand minima. “Early estimates of grand minimum frequency in solar-type stars ranged from 10% to 30%, implying the sun’s influence could be overpowering. More recent studies using data from Hipparcos (a European Space Agency astrometry satellite) and properly accounting for the metallicity of the stars, place the estimate in the range of less than 3%.” This is not a large number, but it is significant. Indeed, the sun could be on the threshold of a mini-Maunder event right now. Ongoing Solar Cycle 24 is the weakest in more than 50 years. Moreover, there is (controversial) evidence of a long-term weakening trend in the magnetic field strength of sunspots. Matt Penn and William Livingston of the National Solar Observatory predict that by the time Solar Cycle 25 arrives, magnetic fields on the sun will be so weak that few if any sunspots will be formed. Independent lines of research involving helio seismology and surface polar fields tend to support their conclusion. (Note: Penn and Livingston were not participants at the NRC workshop.) “If the sun really is entering an unfamiliar phase of the solar cycle, then we must redouble our efforts to understand the sun-climate link,” notes Lika Guhathakurta of NASA’s living with a Star Program, which helped fund the NRC study. “The report offers some good ideas for how to get started.” This image of the Sun's upper photosphere shows bright and dark magnetic structures responsible for variations in TSI. SOURCE: Courtesy of P. Foukal, Heliophysics, Inc. In a concluding panel discussion, the researchers identified a number of possible next steps. Foremost among them was the deployment of a radiometric imager. Devices currently used to measure total solar irradiance (TSI) reduce the entire sun to a single number: the total luminosity summed over all latitudes, longitudes, and wavelengths. This integrated value becomes a solitary point in a time series tracking the sun’s output. In fact, as Peter Foukal of Heliophysics, Inc., pointed out, the situation is more complex. The sun is not a featureless ball of uniform luminosity. Instead, the solar disk is dotted by the dark cores of sunspots and splashed with bright magnetic froth known as faculae. Radiometric imaging would, essentially, map the surface of the sun and reveal the contributions of each to the sun’s luminosity. Of particular interest are the faculae. While dark sunspots tend to vanish during solar minima, the bright faculae do not. This may be why paleo climate records of sun-sensitive isotopes C-14 and Be-10 show a faint 11-year cycle at work even during the Maunder Minimum. A radiometric imager, deployed on some future space observatory, would allow researchers to develop the understanding they need to project the sun-climate link into a future of prolonged spotlessness. Some attendees stressed the need to put sun-climate data in standard formats and make them widely available for multidisciplinary study. Because the mechanisms for the sun’s influence on climate are complicated, researchers from many fields will have to work together to successfully model them and compare competing results. Continued and improved collaboration between NASA, NOAA and the NSF are keys to this process. Hal Maring, a climate scientist at NASA headquarters who has studied the report, notes that “lots of interesting possibilities were suggested by the panelists. However, few, if any, have been quantified to the point that we can definitively assess their impact on climate.” Hardening the possibilities into concrete, physically-complete models is a key challenge for the researchers. Finally, many participants noted the difficulty in deciphering the sun-climate link from paleo climate records such as tree rings and ice cores. Variations in Earth’s magnetic field and atmospheric circulation can affect the deposition of radioisotopes far more than actual solar activity. A better long-term record of the sun’s irradiance might be encoded in the rocks and sediments of the Moon or Mars. Studying other worlds might hold the key to our own. The full report, “The Effects of Solar Variability on Earth’s Climate,” is available from the National Academies Press at http://www.nap.edu/catal og.php?record_id=13519 . ¿Pudo el Bosón de Higgs hacer colapsar el Universo? Cosmólogos británicos han concluido que el Universo pudo no haber durado más de un segundo luego del Big Bang. Foto: El Telescopio BICEP 2 en uno de los dos atardeceres que ocurren en el año en el Polo Sur. El observatorio MAPO (hogar de la Red de telescopios Keck), y la estación del Polo Sur se pueden observar en el fondo. Los cosmólogos británicos están confundidos, ellos predijeron que nuestro Universo no debió durar más de un segundo. Esta extraña conclusión es el resultado de combinar las últimas observaciones del cielo con el reciente descubrimiento del Bosón de Higgs. El Dr. Robert Hogan, del King’s College London (KCL), presentó el trabajo en Junio 24, 2014. En la reunión de la Royal Sociedad Nacional de Astronomía Astronomy, en Portsmouth. Después que nuestro Universo empezó como el Big Bang, se cree que tuvo un corto período de rápida expansión que conocemos como Inflación Cósmica. Aunque algunos detalles de este proceso no son bien entendidos, los cosmólogos han sido capaces de hacer predicciones sobre cómo este proceso afecta al Universo que vemos hoy día. En Marzo de 2014, investigadores colaboradores del BICEP 2 dijeron que habían detectado uno de los efectos predichos. Si es verdad, estos resultados son un gran avance en nuestra comprensión de la Cosmología y confirmación de la Teoría de la Inflación, pero esto ha sido muy controversial y no totalmente aceptado por los cosmólogos. En el estudio, científicos del KCL han investigado lo que las observaciones del BICEP 2 significan para la estabilidad del Universo. Para hacerlo combinaron los resultados con avances recientes en la Física de Partículas. La detección del Bosón de Higgs en el Gran colisionador de Hadrones, anunciado en Julio de 2012: desde entonces se ha aprendido mucho sobre sus propiedades. Medidas del Bosón de Higgs han permitido a los Físicos de Partículas que nuestro Universo se asienta en un “Valle del Campo de Higgs”, el cual describe la manera que otras partículas poseen masa. Sin embargo hay un “Valle” diferente que es mucho más profundo, pero nuestro Universo no cae allí debido a una gran barrera energética. El problema es que los resultados del BICEP 2 predicen que nuestro Universo ha recibido potentes impulsos durante la fase de Inflación, empujándolo al otro “Valle” del Campo de Higgs en una fracción de segundo. Si esto hubiera ocurrido, el Universo habría colapsado en un instante. Robert Hogan, líder del estudio dice que esto es una predicción inaceptable, pues si esto hubiera ocurrido, no estaríamos aquí discutiéndolo. Tal vez los resultados del BICEP 2 contienen un error. Si no, debe haber otros procesos, aun desconocidos, que previnieron que el Universo colapsara. Si los resultados del BICEP 2 son correctos, entonces esto nos dice que existe una nueva e interesante Física de Partículas más allá del modelo estándar. NOTA: Ver: Mensajero 71, Primera Evidencia de la Inflación Mensajero 68, Física de Partículas Mensajero 67, Hawkins y el Origen del Universo Mensajero 66, Bosón de Higgs u otra partícula Mensajero 64, El Bosón de Higgs Ocultación de Marte por la Luna, Julio 06, 2014. Observador: Jorge Luis Salas m / ACA (Asociación Carabobeña de Astronomía) sitio web: 114milimetros.blogspot.com Longitud: -67.960280555556 Latitud: 10.261688888889 Altura: +498.00 metros Huso horario: UTC-4.5 Contacto Tiempo Estimado Tiempo Estimado (Velocidad de la luz infinita) (Velocidad de la luz finita) Contacto 1 (Marte toca la Luna) 2014 JUL 05 22:10:55.84 2014 JUL 05 22:10:45.70 Contacto 1 punto medio (La mitad de Marte esta ocultada) 2014 JUL 05 22:11:51.34 2014 JUL 05 22:11:43.12 Contacto 2 (Marte esta ocultado completamente) 2014 JUL 05 22:12:53.26 2014 JUL 05 22:12:46.99 Contacto 3 (Marte empieza a emerger) 2014 JUL 05 22:27:52.57 2014 JUL 05 22:27:14.77 Contacto 3 punto medio (La mitad de Marte ha emergido) 2014 JUL 05 22:28:51.51 2014 JUL 05 22:28:16.29 Contacto 4 (Fin de la ocultación) 2014 JUL 05 22:29:10.78 2014 JUL 05 22:29:45.15 Duración del evento:+0:16:33.164 horas Trayectoria de Marte Respecto a la Luna: Tiempos de contacto para la latitud local, nótese que será una ocultación rasante. Desapareciendo en T1 Y reapareciendo en T2 Ubicación: San Diego, Carabobo, Venezuela Latitud: N 10° 15’ 42.08’’ Longitud: O 67°57’37.01’’ Altitud (msnm) : 498 Equipo utilizado: Binocular TASCO 10X70 Telescopio reflector newtoniano Celestron Firstscope 76/300 Oculares: 4mm 9mm 20mm 32 mm Filtro lunar Celestron Procedimiento: Visual con binocular y Telescopio Software/ application: GPS STATUS, Time the sat, cronómetro Datos de Observación : Contacto TC. HLV (T.U. – 4,5h) C-1 Marte toca la Luna 22:10:11,778 C 2 ½ Marte ocultado 22:11:27,340 C-3 Marte Ocultado 22:12:39,000 C-4 Marte Sale 22:27:34,795 C-5 ½ Marte emerge 22:28:59,000 C-6 Fin de la Ocultación 22:30:14,540 En Caracas estuvo nublado y no se realizaron observaciones, solo Carlos Quintana pudo medir La gran Mancha Roja de Júpiter está más pequeña que nunca antes NASA La Mancha Roja, un ícono del planeta Júpiter, la cual es un sistema de alta presión circular más grande que nuestro planeta, se ha empequeñecido al menor tamaño jamás medido. Imagen: NASA, ESA, and A. Simon (Goddard Space Flight Center) Los astrónomos han seguido el empequeñecimiento de la Gran Mancha Roja de Júpiter desde la década de los años 30. Mediciones recientes realizadas por el Telescopio Espacial Hubble confirman que esta tiene ahora unas 10.250 millas de diámetro, el tamaño más pequeño jamás medido, según Amy Simon del Goddard Space Flight Center de la NASA en Greenbelt, Md. Mediciones históricas que van para atrás hasta los últimos años del siglo XIX (1.800´s), muestran una Gran Mancha Roja de hasta 25.500 millas de diámetro en su eje mayor. Las sondas Voyager 1 y Voyager 2 midieron un diámetro de 14.500 millas en 1979. Comenzando en el 2012, observaciones de aficionados revelaron un notable incremento en la taza de encogimiento. El talle de la GMR se está encogiendo 580 millas por año, y el óvalo de la GMR paso a ser un circulo. La causa del encogimiento aún no se ha explicado. En observaciones recientes se han observado pequeños remolinos se están alimentando de la tormenta. Se piensa que ellas son responsables de los cambios acelerados por alteración de la dinámica y energía interna de la Gran Mancha Roja. El equipo de astrónomos liderizado por A. Simon, planea estudiar los pequeños remolinos y estudiar la dinámica de la GMR para determinar si ellos pueden alimentar o frenar el momentum al entrar en el vórtice. Comparaciones realizadas con el Hubble tomadas en 1995, cuando el eje mayor de la GMR era 13.020 millas, con mediciones del 2009, muestran que en este año medía 11.130 millas. Unas 1890 millas menos. Pero Júpiter es un planeta muy activo en su atmósfera, hace 3 años la Banda Ecuatorial Sur desapareció u estuvo 2 años ausente. Algo así podría estar pasando a la Gran Mancha Roja, o tal vez este rasgo distintivo del planeta este llegando a su fin. Aún hay mucho que investigar. La acidificación actual del mar es mucho más rápida que la de hace 56 millones de años La acidificación actual del mar es mucho más rápida que la de hace 56 millones de años Hace unos 56 millones de años, hubo un período de calentamiento global abrupto, el cual se conoce como el Máximo Térmico del Paleoceno-Eoceno (MTPE, o PETM por sus siglas en inglés). Durante esta etapa geológica, un pulso masivo de dióxido de carbono emitido hacia la atmósfera elevó ostensiblemente las temperaturas a escala global. En los océanos, los sedimentos del carbonato se disolvieron, algunas especies se extinguieron y otras experimentaron un fuerte cambio de rumbo evolutivo. Lejos de ser un fenómeno de interés exclusivo para los estudiosos del pasado, el Máximo Térmico del Paleoceno-Eoceno es hoy en día un tema de la máxima actualidad, ya que cada vez está más claro que se trata del análogo más cercano, por similitud y por cercanía en el tiempo, al actual calentamiento global. Entre los efectos comunes a ambos episodios figura la acidificación oceánica. La comunidad científica ha sospechado desde hace mucho tiempo que fue la acidificación oceánica la ejecutora de los cambios nocivos en el mar que perjudicaron a los antiguos ecosistemas marinos. Aquella crisis medioambiental aparece marcada claramente en los registros fósiles y geológicos. De manera similar a lo que ocurre hoy, la creciente abundancia del dióxido de carbono propició que éste se combinase con el agua salada de los océanos de tal modo que alteró las propiedades químicas de ésta. Ahora unos científicos han logrado cuantificar por primera vez la magnitud de la acidificación de la superficie oceánica durante el Máximo Térmico del Paleoceno-Eoceno, y las noticias no son buenas: Nuestros océanos actuales están en camino de acidificarse tanto o más que en aquel entonces, sólo que a una velocidad mucho más rápida, que puede ser hasta 10 veces más veloz que en esa época de referencia. [Img #20956] Los foraminíferos de la especie Aragonia velascoensis se extinguieron, junto con otras criaturas marinas, hace unos 56 millones de años, por culpa de la acidificación oceánica, rápida para lo que el ritmo de la evolución es capaz de afrontar, pero que pese a todo fue mucho más lenta que la actual. (Foto: Ellen Thomas / Universidad Yale) El equipo de la paleoceanógrafa Bärbel Hönisch, del Observatorio Terrestre Lamont-Doherty, adscrito a la Universidad de Columbia, en la ciudad de Nueva York, y Ellen Thomas, de la Universidad Yale en New Haven, Connecticut, todas estas entidades en Estados Unidos, estima que la acidez oceánica aumentó en aproximadamente un 100 por cien a lo largo de un periodo de unos mil años o más, y se quedó así durante los siguientes 70.000 años. En este ambiente alterado radicalmente, algunas especies se extinguieron inexorablemente mientras que otras se adaptaron y evolucionaron. Los océanos, cual héroes silenciosos de nuestros tiempos, han absorbido cerca de un tercio de las emisiones de carbono que los humanos hemos bombeado a la atmósfera desde la industrialización. Con su acción protectora, han ayudado a mantener la temperatura más baja de lo que habría ya llegado a ser en su ausencia. Pero esa captura del carbono tiene su precio. Las reacciones químicas causadas por ese exceso de CO2 han hecho que el agua de mar sea más ácida, desposeyéndola de los iones de carbonato que corales, moluscos y algunas especies de plancton necesitan para desarrollar sus conchas y esqueletos. En los últimos 150 años, el pH de los océanos ha descendido de manera significativa (o sea que su agua se ha vuelto más ácida). Se estima que desde ahora y hasta finales de este siglo, la caída del pH oceánico será incluso mayor que la registrada en el último siglo y medio. Sumando la caída de los últimos 150 años con la pronosticada para el siglo actual, el aumento de acidez marina resultante es un poco mayor que el estimado para todo el Máximo Térmico del Paleoceno-Eoceno. Lo más inquietante, sin embargo, es que, mientras que el cambio de pH en el Máximo Térmico del Paleoceno-Eoceno se obró a lo largo de unos mil años, el actual cambio de pH, si se cumplen las previsiones, se habrá obrado en un periodo mucho menor, de tan solo unos 250 años. El Spitzer de NASA, WISE Encuentra un Sol vecino cercano y frío. NASA Esta concepción artística muestra al objeto llamado WISE J085510.83-071442.5, la más fría enana marrón conocida. Estas estrellas son pequeños cuerpos parecidos a estrellas que carecen de masa para quemar sus combustibles nucleares. El NASA's Wide-field Infrared Survey Explorer (WISE) y el Spitzer Space Telescope han descubierto lo que parece ser la más fría estrella enana marrón conocida, una estrella muy tenue que sorprendentemente es tan fría como los polos terrestres. Imágenes de telescopios espaciales también descubrieron que su distancia es de solo 7.2 años luz, lo que la coloca entre los 4 sistemas estelares más cercanos a nuestro Sol. El sistema más cercano es un trío de estrellas que llamamos Alfa Centauro y que dista a solo 4.3 años luz de nosotros. "Es muy excitante descubrir un nuevo vecino tan cercano a nuestro Sistema Solar”, dice Kevin Luhman, un astrónomo de la Universidad de la Pennsyvania State Universitys University Park Center para Exoplanetas y Mundos Habitable, "Y dada su temperatura extrema, nos puede decir mucho sobre las atmósferas planetarias, las cuales poseen frecuentemente, temperaturas similares”. Las estrellas Enanas Marrones comienzan sus vidas como estrellas, bolas de gas que colapsan, pero por no poseer masa suficiente para encender sus hornos nucleares no pueden radiar energía y brillar como estrellas. La nueva estrella Enana Marrón, la más fría jamás descubierta es llamada: WISE J085510.83-071442.5. Ella tiene una fría temperatura entre -54 y 9º Fahrenheit, (-48 y 9º Celsius). Los records anteriores para la enana marrón más fría, también descubierta por el WISE y el Spitzer, era como la temperatura de una habitación normal. El WISE fue capaz de captar el raro objeto por realizar dos surveys del cielo en infrarrojo, observando áreas hasta 3 veces. Objetos fríos como las enanas marrones pueden ser invisibles al observarlas con telescopios de luz visible, pero su brillo térmico aparece en IR aunque sean objetos fríos. En adición, mientras más cercano sea un objeto, más parecerá moverse en imágenes tomadas tras varios meses. Este objeto se movía mucho en los datos del WISE, lo que nos dijo que era muy cercano. Luego de notar el rápido movimiento del WISE J085510.83-071442.5 en Marzo de 2013, Luhman analizó imágenes de Spitzer y el Telescopio Géminis Sur en cerro Pachón, chile. Las observaciones del telescopio infrarrojo Spitzer, ayudaron a determinar la helada temperatura de la enana roja. Combinando las observaciones de WISE y Spitzer, tomadas en posiciones diferentes alrededor del Sol, realizaron la medición de distancias por efecto de la Paralaje. Este es el mismo principio que explica el movimiento de un dedo de su mano, cuando lo coloca frente a su rostro y lo mira con el ojo derecho y luego el izquierdo. Es interesante que después de décadas estudiando el cielo, aún no poseamos in inventario completo de los vecinos más cercanos al Sol, de acuerdo a lo dicho por Michael Werner, científico del proyecto Spitzer, del Jet Propulsion Laboratory, de NASA, en Pasadena, California. El JPL gerencia y maneja el Spitzer. Este nuevo resultado es excitante, pues demuestra el poder de la exploración astronómica utilizando nuevas herramientas como los telescopios IR Wise y Spitzer. El WISE J085510.83-071442.5 se estima que tiene entre 3 y 10 masas de Júpiter. Con esta masa debe ser un gigante gaseoso igual al planeta, que fue eyectado de su sistema estelar. Algunos científicos creen sin embargo que se trata de un estrella Enana Marrón y no de un planeta, pues se sabe que estas son muy comunes. Si es así es una de las Enanas Marrones con menos masa conocida. En Marzo del 2013, los análisis imágenes de Luhman realizados con el WISE descubrió un par de enanas marrones más calientes, a una distancia de 6.5 Años Luz, haciendo a este sistema el tercero más cercano al Sol. Su búsqueda de objetos rápidos también demostró que el sistema solar exterior probablemente no posee un planeta grande no descubierto y al cual se ha llamado Planeta X o Némesis. A New View of the Red Planet Authors: Tenielle Gaither, [email protected]; Kenneth Tanaka, [email protected]; James Skinner, [email protected]; Jennifer LaVista, [email protected] Get ready, because now you can explore the most comprehensive representation of Mars with a new global geologic map created by the U.S. Geological Survey. This new view of the “Red Planet’s” surface provides a framework for continued scientific investigation of Mars as the long-range target for human space exploration. What Does the New Map Show? The USGS-led mapping effort reveals that the Martian surface is generally older than previously thought. Three times as much surface area dates to the first major geologic time period – the Early Noachian Epoch – than was previously mapped. This timeframe is the earliest part of the Noachian Period, which ranges from about 4.1 to about 3.7 billion years ago, and was characterized by high rates of meteorite impacts, widespread erosion of the Martian surface and the likely presence of abundant surface water. The map also confirms previous work that suggests Mars had been geologically active until the present day. There is evidence that major changes in Mars’ global climate supported the temporary presence of surface water and near-surface groundwater and ice. These changes were likely responsible for many of the major shifts in the environments where Martian rocks were formed and subsequently eroded. This new map will serve as a key reference for the origin, age and historic change of geological materials anywhere on Mars. Why Explore Space? For hundreds of years, geologic maps have helped drive scientific thought. This new global geologic map of Mars, as well as the recent global geologic maps of Jupiter’s moons Ganymede and Io, also illustrates the overall importance of geologic mapping as an essential tool for the exploration of the solar system. “Spacecraft exploration of Mars over the past couple decades has greatly improved our understanding of what geologic materials, events and processes shaped its surface,” said USGS scientist and lead author, Dr. Kenneth Tanaka. “The new geologic map brings this research together into a holistic context that helps to illuminate key relationships in space and time, providing information to generate and test new hypotheses.” Out of this World Science Takes Time The new map brings together observations and scientific findings from four orbiting spacecraft that have been acquiring data for more than 16 years. The result is an updated understanding of the geologic history of the surface of Mars – the solar system’s most Earth-like planet and the only other one in our Sun’s “habitable zone.” The Martian surface has been the subject of scientific observation since the 1600s, first by Earth-based telescopes, and later by fly-by missions and orbiting spacecraft. The Mariner 9 and Viking Orbiter missions produced the first planet-wide views of Mars’ surface, enabling publication of the first global geologic maps (in 1978 and 1986-87, respectively) of a planetary surface other than the Earth and the Moon. A new generation of sophisticated scientific instruments flown on the Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter spacecraft has provided diverse, high quality data sets that enable more sophisticated remapping of the global-scale geology of Mars. How the USGS Got Involved in Space Science The production of planetary cartographic products has been a focal point of research at the USGS Astrogeology Science Center since its inception in the early 1960s. The USGS began producing planetary maps in support of the Apollo Moon landings, and continues to help establish a framework for integrating and comparing past and future studies of extraterrestrial surfaces. In many cases, these planetary geologic maps show that, despite the many differences between bodies in our solar system, there are many notable similarities that link the evolution and fate of our planetary system together. The mission of the USGS Astrogeology Science Center is to serve the nation, the international planetary science community and the general public’s pursuit of new knowledge of our solar system. The team’s vision is to be a national resource for the integration of planetary geosciences, cartography and remote sensing. As explorers and surveyors with a unique heritage of proven expertise and international leadership, USGS astrogeologists enable the ongoing successful investigation of the solar system for humankind. Enabling Future Exploration “Findings from the map will enable researchers to evaluate potential landing sites for future Mars missions that may contribute to further understanding of the planet’s history,” said USGS Acting Director Suzette Kimball. “The new Mars global geologic map will provide geologic context for regional and local scientific investigations for many years to come.” The project was funded by NASA through its Planetary Geology and Geophysics Program. Violenta historia del Sol joven resuelve misterio de los Meteoritos ESA Un grupo de astrónomos ha empleado el telescopio espacial Herschel de ESA para estudiar los violentos comienzos de una estrella tipo Sol, encontrando indicios de potentes vientos estelares que podrían resolver un extraño misterio sobre meteoritos. A pesar de su tranquila apariencia en el cielo nocturno, las estrellas son hornos abrasadores que llegan a la vida a través de procesos tumultuosos, y nuestro Sol, de 4500 millones de años de edad, no es una excepción. Para conocer un poco más sobre sus duros inicios, los astrónomos recogen pistas, no sólo en el Sistema Solar, sino también estudiando estrellas jóvenes en otros lugares de nuestra Galaxia. Empleando Herschel para estudiar la composición química de regiones donde las estrellas están naciendo hoy en día, un equipo de astrónomos ha observado que un objeto en particular es diferente. La fuente inusual es un prolífico vivero estelar llamado OMC2 FIR4, una agrupación de estrellas nuevas situadas en el interior de una nube gaseosa y polvorienta, cerca de la famosa Nebulosa de Orión. "Para nuestra sorpresa, descubrimos que la proporción entre dos especies químicas, una basada en el carbono y oxígeno y la otra en el nitrógeno, es mucho más pequeña en este objeto que en cualquier otra protoestrella que conozcamos", afirma la Dra. Cecilia Ceccarelli, quien dirigió el estudio."La causa más probable en este ambiente es un violento viento de partículas muy energéticas, expulsado por lo menos por una de las estrellas embrionarias que están tomando forma en este huevo protoestelar", añade la Dra. Ceccarelli. Los astrónomos piensan que un viento violento parecido de partículas también barrió el Sistema Solar primitivo, y este descubrimiento podría finalmente constituir una explicación al origen de un elemento químico particular observado en meteoritos, el berilio 10. Junio fue el mes más cálido en la Tierra desde 1880 NOAA Imagen de la Tierra desde la Estación Espacial Internacional La, Administración Nacional Atmosférica y Oceánica de EE UU registró que la media del planeta se colocó en 15,5°C. El mes de junio registró las temperaturas globales más cálidas desde que comenzaron los registros en 1880, al superar la media de 15,5°C (59,9°F) por 0,72°C (1,30°F), informó hoy la Administración Nacional Atmosférica y Oceánica de EEUU, (NOAA, por sus siglas en Inglés), en su reporte mensual. Tanto la temperatura de la superficie terrestre como la de los océanos alcanzaron temperaturas superiores a la media. No obstante, Jessica Blunden, científica de la NOAA, apuntó que "el calentamiento fue impulsado por las temperaturas récord en el océano", y agregó que parte de esta subida se debió al comienzo del fenómeno de El Niño, el calentamiento de las aguas tropicales del Pacífico. La de la tierra fue 0,95°C (1,71°F) mayor que la media de 13,3°C (55,9°F), y se situó como el séptimo junio más cálido; mientras que la del océano fue de 0,64°C (1,15°F) por encima de la media de 16,4°C (61,5°F), y se convierte así en el junio más cálido desde que se empezaron a compilar datos. Estos datos reflejan también un repunte en las temperaturas globales en los primeros seis meses del año: las combinadas de tierra y mar fueron 0,67°C (1,21°F) superiores a la media de 13,5°C (56,3°F), las terceras más altas para un período enero-junio desde 1880. Esta alza en las temperaturas se produjo de manera general en todo el mundo, ya que se batieron récords en Groenlandia, el norte de Sudamérica, el este y centro de África y el sudeste asiático, así como en Nueva Zelanda. La agencia federal de EE.UU. recordó en su reporte mensual que nueve de los diez meses de junio más cálidos registrados han tenido lugar en el siglo XXI. Asimismo, indicó que el último mes de junio por debajo de la media se produjo en 1976. Sonda Voyager 1 podría no haber alcanzado el espacio interestelar. En 2012, el equipo de la misión Voyager anunció que la nave Voyager 1 había pasado al espacio interestelar, viajando más lejos de lo que lo ha hecho cualquier objeto de fabricación humana. Pero en los casi dos años que han transcurrido desde ese anuncio histórico, y a pesar de las observaciones posteriores que lo respaldaban, continúa la incertidumbre acerca de si la Voyager 1 realmente cruzó la frontera. Hay algunos científicos que dicen que la nave espacial todavía se encuentra dentro de la heliosfera (la región del espacio dominada por el Sol y su viento de partículas energéticas) y que aún no ha alcanzado el espacio entre las estrellas. Ahora, dos científicos del equipo de la Voyager han desarrollado una prueba que podría demostrar de una vez por todas si la Voyager 1 ha cruzado la frontera. Los científicos predicen que en los próximos dos años Voyager 1 cruzará la capa de corriente eléctrica (la superficie dentro de la heliosfera donde la polaridad del campo magnético del Sol cambia de positiva a negativa). La nave detectará una inversión del campo magnético, demostrando que todavía se encuentra dentro de la heliosfera. Pero si la inversión del campo magnético no se produce dentro de un año o dos, tal como se espera, eso confirmaría que Voyager 1 ya ha pasado al espacio interestelar. Las naves espaciales Voyager 1 y 2 fueron lanzadas en 1977 hacia Júpiter y Saturno. Desde entonces la misión se ha extendido a la exploración de los límites más exteriores de la influencia del Sol y aún más allá. Voyager 2, que también pasó por Urano y Neptuno, está de camino al espacio interestelar. La supernova más brillante de la historia A 7.000 años luz de la Tierra, era tan espectacular que pudo ser contemplada durante más de tres años en el siglo XI. Ahora, los científicos saben qué la provocó NASA/NRAO/Middlebury College El remanente de supernova, a 7.000 años luz de la Tierra Una investigación, en la que ha participado el Consejo Superior de Investigaciones Científicas (CSIC), ha descubierto el origen del que hasta ahora se considera el "evento estelar más brillante" que ha podido ser contemplado en la historia desde la Tierra, la supernova SN1006, que tuvo lugar en el año 1006 a unos 7.000 años luz de la Tierra, fruto de la fusión de dos enanas blancas, según ha publicado la revista Nature en su portada. De esta forma, el CSIC señala que este evento estelar se clasifica dentro de las supernovas de tipo Ia, que son aquellas generadas por sistemas binarios en los que dos objetos astronómicos están ligados entre sí por su fuerza gravitatoria. Asimismo, apunta que el estudio calcula que la luz emitida por SN1006 fue equivalente a "una cuarta parte" de la del brillo de la Luna, lo que respaldaría los registros históricos de astrólogos de la época que indican que la explosión fue visible en distintas partes del mundo durante "más de tres años" y que fue "aproximadamente" tres veces más brillante que Venus. Por otro lado, explica que "usualmente" estos sistemas suelen estar formados por una enana blanca y una estrella normal que le aporta la materia necesaria para alcanzar la "masa crítica" de 1,4 veces la del Sol y, una vez alcanzada, la enana blanca comienza la fusión de su núcleo que origina una explosión termonuclear. No obstante, ha apuntado que "también existe la posibilidad de que la supernova se origine a causa de la fusión de dos enanas blancas conectadas entre sí". Por su parte, la investigadora del Instituto de Física Fundamental del CSIC Pilar Ruiz-Lapuente, que ha participado en este estudio, ha manifestado que "la exploración en torno al lugar donde se produjo la supernova SN1006 no ha detectado a ningún candidato a compañero de la enana blanca original, lo que invita a pensar que probablemente se produjo mediante la fusión de dos enanas blancas conectadas entre sí". Ante esto, el investigador del Instituto de Astrofísica de Canarias Jonay González, que ha liderado el trabajo, ha argumentado que "existen tres tipos de estrellas en la región donde tuvo lugar la explosión, las gigantes, sub gigantes y enanas, pero las observaciones sólo detectaron cuatro estrellas gigantes situadas a la misma distancia que el remanente de la supernova". Sin dejar pistas Así, ha planteado que "las simulaciones numéricas no predicen a una compañera de estas características, las cualidades de una posible estrella compañera". En este sentido, Ruiz-Lapuente ha indicado que "tras la explosión de la supernova, la estrella compañera de la enana blanca se asemejaría más a una estrella de helio, pero ninguna de este tipo fue detectada en la región de estudio por lo que se desprende que el origen de SN1006 tuvo lugar en la colisión de dos enanas blancas, cuyo material fue expulsado sin dejar ningún testigo de la explosión". Por último, la investigadora del CSIC ha apuntado que "hasta la fecha se habían encontrado algunas supernovas extra galácticas que no mostraban ninguna señal de la existencia de la estrella compañera". Por ello, considera que estos "nuevos resultados, junto con otros anteriores, suponen que la fusión de enanas blancas podría ser una vía usual para dar lugar a estas violentas explosiones termonucleares". En el año 2004, Ruiz-Lapuente ya dirigió la investigación para descubrir el origen de la supernova del año 1572, donde hallaron la estrella que acompañó a la enana blanca que provoco este evento estelar. Near- and Mid-Infrared Photometry of the Pleiades and a New List of Substellar Candidate Members1,2 John R. Stauffer Spitzer Science Center, Caltech 314-6, Pasadena, CA 91125; [email protected] Lee W. Hartmann Astronomy Department, University of Michigan Giovanni G. Fazio , Lori E. Allen , and Brian M. Patten Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Patrick J. Lowrance , Robert L. Hurt , and Luisa M. Rebull Spitzer Science Center, Caltech, Pasadena, CA 91125 Roc M. Cutri and Solange V. Ramirez Infrared Processing and Analysis Center, Caltech 220-6, Pasadena, CA 91125 3 Erick T. Young , George H. Rieke , Nadya I. Gorlova , and James C. Muzerolle Steward Observatory, University of Arizona, Tucson, AZ 85726 Cathy L. Slesnick Astronomy Department, Caltech, Pasadena, CA 91125 Michael F. Skrutskie Astronomy Department, University of Virginia, Charlottesville, VA 22903 ABSTRACT We make use of new near- and mid-IR photometry of the Pleiades cluster in order to help identify proposed cluster members. We also use the new photometry with previously published photometry to define the single-star mainsequence locus at the age of the Pleiades in a variety of color-magnitude planes. The new near- and mid-IR photometry extend effectively 2 mag deeper than the 2MASS All-Sky Point Source catalog, and hence allow us to select a new set of candidate very low-mass and substellar mass members of the Pleiades in the central square degree of the cluster. We identify 42 new candidate members fainter than Ks = 14 (corresponding to 0.1 M ). These candidate members should eventually allow a better estimate of the cluster mass function to be made down to of order 0.04 M . We also use new IRAC data, in particular the images obtained at 8 m, in order to comment briefly on interstellar dust in and near the Pleiades. We confirm, as expected, that—with one exception—a sample of low-mass stars recently identified as having 24 m excesses due to debris disks do not have significant excesses at IRAC wavelengths. However, evidence is also presented that several of the Pleiades high-mass stars are found to be impacting with local condensations of the molecular cloud that is passing through the Pleiades at the current epoch. Subject headings: open clusters and associations: individual (Pleiades); stars: low-mass, brown dwarfs Online material: color figure, machine-readable tables 1 This work is based (in part) on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. 2 This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. 3 Current address: University of Florida, 211 Bryant Space Center, Gainesville, FL 32611. 1. INTRODUCTION Because of its proximity, youth, richness, and location in the northern hemisphere, the Pleiades has long been a favorite target of observers. The Pleiades was one of the first open clusters to have members identified via their common proper motion (Trumpler 1921), and the cluster has since then been the subject of more than a dozen proper-motion studies. Some of the earliest photoelectric photometry was for members of the Pleiades (Cummings 1921), and the cluster has been the subject of dozens of papers providing additional optical photometry of its members. The youth and nearness of the Pleiades make it a particularly attractive target for identifying its substellar population, and it was the first open cluster studied for those purposes (Jameson & Skillen 1989; Stauffer et al. 1989). More than 20 papers have been subsequently published, identifying additional substellar candidate members of the Pleiades or studying their properties. We have three primary goals for this paper. First, while extensive optical photometry for Pleiades members is available in the literature, photometry in the near- and mid-IR is relatively spotty. We will remedy this situation by using new 2MASS JHKs and Spitzer Infrared Array Camera (IRAC) photometry for a large number of Pleiades members. We will use these data to help identify cluster nonmembers and to define the single-star locus in color-magnitude diagrams for stars of 100 Myr age. Second, we will use our new IR imaging photometry of the center of the Pleiades to identify a new set of candidate substellar members of the cluster, extending down to stars expected to have masses of order 0.04 M . Third, we will use the IRAC data to briefly comment on the presence of circumstellar debris disks in the Pleiades and the interaction of the Pleiades stars with the molecular cloud that is currently passing through the cluster. In order to make best use of the IR imaging data, we will begin with a necessary digression. As noted above, more than a dozen proper-motion surveys of the Pleiades have been made in order to identify cluster members. However, no single catalog of the cluster has been published that attempts to collect all of those candidate members in a single table and cross-identify those stars. Another problem is that, while there have been many papers devoted to providing optical photometry of cluster members, that photometry has been bewilderingly inhomogeneous in terms of the number of photometric systems used. In § 3 and in the Appendix, we describe our efforts to create a reasonably complete catalog of candidate Pleiades members and to provide optical photometry transformed to the best of our ability onto a single system. 2. NEW OBSERVATIONAL DATA 2.1. 2MASS "6x" Imaging of the Pleiades During the final months of Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006) operations, a series of special observations were carried out that employed exposures 6 times longer than used for the primary survey. These socalled "6x" observations targeted 30 regions of scientific interest including a 3 × 2 area centered on the Pleiades cluster. The 2MASS 6x data were reduced using an automated processing pipeline similar to that used for the main survey data, and a calibrated 6x Image Atlas and extracted 6x Point and Extended Source Catalogs (6x-PSC and 6x-XSC) analogous to the 2MASS All-Sky Atlas, PSC, and XSC have been released as part of the 2MASS Extended Mission. A description of the content and formats of the 6x image and catalog products, and details about the 6x observations and data reduction, are given in § A3 of the 2MASS Explanatory Supplement by Cutri et al.4 The 2MASS 6x Atlas and Catalogs may be accessed via the online services of the NASA/IPAC Infrared Science Archive.5 Figure 1 shows the area on the sky imaged by the 2MASS 6x observations in the Pleiades field. The region was covered by two rows of scans, each scan being 1° long (in declination) and 8.5 wide in right ascension. Within each row, the scans overlap by approximately 1 in right ascension. There are small gaps in coverage in the declination boundary between the rows, and one complete scan in the southern row is missing because the data in that scan did not meet the minimum required photometric quality. The total area covered by the 6x Pleiades observations is approximately 5.3 deg2. Fig. 1 Spatial coverage of the 6 times deeper (270 "2MASS 6x" observations of the Pleiades. The kB) 2MASS survey region is approximately centered on Alcyone, the most massive member of the Pleiades. The trapezoidal box roughly indicates the region covered with the shallow IRAC survey of the cluster core. The star symbols correspond to the brightest B star members of the cluster. The red points are the location of objects in the 2MASS 6x Point Source Catalog. There are approximately 43,000 sources extracted from the 6x Pleiades observations in the 2MASS 6x-PSC, and nearly 1500 in the 6x-XSC. Because there are at most about 1000 Pleiades members expected in this region, only 2% of the 6x-PSC sources are cluster members, and the rest are field stars and background galaxies. The 6x-XSC objects are virtually all resolved background galaxies. Near-infrared color-magnitude and colorcolor diagrams of the unresolved sources from the 2MASS 6x-PSC and all sources in the 6x-XSC sources from the Pleiades region are shown in Figures 2 and 3, respectively. The extragalactic sources tend to be redder than most stars, and the galaxies become relatively more numerous toward fainter magnitudes. Unresolved galaxies dominate the point sources that are fainter than Ks > 15.5 and redder than J - Ks > 1.2 mag. Fig. 2 Color-magnitude diagram for the Pleiades derived from the 2MASS 6x observations. The red dots correspond to objects identified as unresolved, whereas the green dots correspond to extended sources (primarily background galaxies). The lack of green dots fainter than K = 16 is indicative that too few photons are available to identify sources as extended—the extragalactic population presumably increases to fainter magnitudes. Fig. 3 Same as Fig. 2, except in this case the axes are J - H and H - Ks. The extragalactic objects are very red in both colors. The 2MASS 6x observations were conducted using the same freeze-frame scanning technique used for the primary survey (Skrutskie et al. 2006). The longer exposure times were achieved by increasing the "READ2-READ1" integration to 7.8 s from the 1.3 s used for primary survey. However, the 51 ms "READ1" exposure time was not changed for the 6x observations. As a result, there is an effective "sensitivity gap" in the 8–11 mag region where objects may be saturated in the 7.8 s READ2-READ1 6x exposures, but too faint to be detected in the 51 ms READ1 exposures. Because the sensitivity gap can result in incompleteness and/or flux bias in the photometric overlap regime, the near-infrared photometry for sources brighter than J = 11 mag in the 6x-PSC was taken from the 2MASS All-Sky PSC during compilation of the catalog of Pleiades candidate members presented in Table 2 (see § 3). 4 See http://www.ipac.caltech.edu/2mass/releases/allsky/doc/explsup.html. 5 See http://irsa.ipac.caltech.edu. 2.2. Shallow IRAC Imaging Imaging of the Pleiades with Spitzer was obtained in 2004 April as part of a joint GTO program conducted by the IRAC instrument team and the Multiband Imaging Photometer for Spitzer (MIPS) instrument team. Initial results of the MIPS survey of the Pleiades have already been reported in Gorlova et al. (2006). The IRAC observations were obtained as two astronomical observing requests (AORs). One of them was centered near the cluster center, at R.A. = 03h47m00.0s and decl. = 24 07 (J2000.0), and consisted of a 12 row by 12 column map, with "frame times" of 0.6 and 12.0 s and two dithers at each map position. The map steps were 290 in both the column and row direction. The resultant map covers a region of approximately 1 deg2, and a total integration time per position of 24 s over most of the map. The second AOR used the same basic mapping parameters, except it was smaller (9 rows by 9 columns) and was instead centered northwest from the cluster center at R.A. = 03h44m36.0s and decl. = 25 24 . A two-band color image of the AOR covering the center of the Pleiades is shown in Figure 4. A pictorial guide to the IRAC image providing Greek names for a few of the brightest stars, and Hertzsprung (1947) numbers for several stars mentioned in § 6 is provided in Figure 5. Fig. 4 Two-color (4.5 and 8.0 m) mosaic of the central square degree of the Pleiades from the IRAC survey. North is approximately vertical, and east is approximately to the left. The bright star nearest the center is Alcyone; the bright star at the left of the mosaic is Atlas; and the bright star at the right of the mosaic is Electra. Fig. 5 Finding chart corresponding approximately to the region imaged with IRAC. The large, five-pointed stars are all of the Pleiades members brighter than V = 5.5. The small open circles correspond to other cluster members. Several stars with 8 m excesses are labeled by their HII numbers and are discussed further in § 6. The short lines through several of the stars indicate the size and position angle of the residual optical polarization (after subtraction of a constant foreground component), as provided in Fig. 6 of Breger (1986). We began our analysis with the basic calibrated data (BCDs) from the Spitzer pipeline, using the S13 version of the Spitzer Science Center pipeline software. Artifact mitigation and masking was done using the IDL tools provided on the Spitzer contributed software Web site. For each AOR, the artifact-corrected BCDs were combined into single mosaics for each channel using the post-BCD "MOPEX" package (Makovoz & Marleau 2005). The mosaic images were constructed with 1.22 × 1.22 pixels (i.e., approximately the same pixel size as the native IRAC arrays). We derived aperture photometry for stars present in these IRAC mosaics using both APEX (a component of the MOPEX package) and the "phot" routine in DAOPHOT. In both cases, we used a 3 pixel radius aperture and a sky annulus from 3 to 7 pixels (except that for channel 4, for the phot package we used a 2 pixel radius aperture and a 2–6 pixel annulus because that provided more reliable fluxes at low flux levels). We used the flux for zero-magnitude calibrations provided in the IRAC data handbook (280.9, 179.7, 115.0, and 64.1 Jy for channels 1–4, respectively), and the aperture corrections provided in the same handbook (multiplicative flux correction factors of 1.124, 1.127, 1.143, and 1.584 for channels 1–4, inclusive. The channel 4 correction factor is much bigger because it is for an aperture radius of 2 rather than 3 pixels.). Figures 6 and 7 provide two means to assess the accuracy of the IRAC photometry. The first figure compares the aperture photometry from APEX to that from phot and shows that the two packages yield very similar results when used in the same way. For this reason, we have simply averaged the fluxes from the two packages to obtain our final reported value. The second figure shows the difference between the derived 3.6 and 4.5 m magnitudes for Pleiades members. Based on previous studies (e.g., Allen et al. 2004), we expected this difference to be essentially zero for most stars, and the Pleiades data corroborate that expectation. For [3.6] < 10.5, the rms dispersion of the magnitude difference between the two channels is 0.024 mag. Assuming that each channel has similar uncertainties, this indicates an internal 1 accuracy of order 0.017 mag. The absolute calibration uncertainty for the IRAC fluxes is currently estimated at of order 0.02 mag. Figure 7 also shows that fainter than [3.6] = 10.5 (spectral type later than about M0), the [3.6] [4.5] color for M dwarfs departs slightly from zero, becoming increasingly redder to the limit of the data (about M6). Fig. 6 Comparison of aperture photometry for Pleiades members derived from the IRAC 3.6 m mosaic using the Spitzer APEX package and the IRAF implementation of DAOPHOT. Fig. 7 Difference between aperture photometry for Pleiades members for IRAC channels 1 and 2. The [3.6] - [4.5] color begins to depart from essentially zero at magnitudes of 10.5, corresponding approximately to spectral type M0 in the Pleiades. 3. A CATALOG OF PLEIADES CANDIDATE MEMBERS If one limits oneself to only stars visible with the naked eye, it is easy to identify which stars are members of the Pleiades—all of the stars within a degree of the cluster center that have V < 6 are indeed members. However, if one were to try to identify the M dwarf stellar members of the cluster (roughly 14 < V < 23), only of order 1% of the stars toward the cluster center are likely to be members, and it is much harder to construct an uncontaminated catalog. The problem is exacerbated by the fact that the Pleiades is old enough that mass segregation through dynamical processes has occurred, and therefore one has to survey a much larger region of the sky in order to include all of the M dwarf members. The other primary difficulty in constructing a comprehensive member catalog for the Pleiades is that the pedigree of the candidates varies greatly. For the best-studied stars, astrometric positions can be measured over temporal baselines ranging up to a century or more, and the separation of cluster members from field stars in a vector point diagram (VPD) can be extremely good. In addition, accurate radial velocities and other spectral indicators are available for essentially all of the bright cluster members, and these further allow membership assessment to be essentially definitive. Conversely, at the faint end (for stars near the hydrogen-burning mass limit in the Pleiades), members are near the detection limit of the existing wide-field photographic plates, and the errors on the proper motions become correspondingly large, causing the separation of cluster members from field stars in the VPD to become poor. These stars are also sufficiently faint that spectra capable of discriminating members from field dwarfs can only be obtained with 8 m class telescopes, and only a very small fraction of the faint candidates have had such spectra obtained. Therefore, any comprehensive catalog created for the Pleiades will necessarily have stars ranging from certain members to candidates for which very little is known and where the fraction of spurious candidate members increases to lower masses. In order to address the membership uncertainties and biases, we have chosen a sliding scale for inclusion in our catalog. For all stars, we require that the available photometry yields location in color-color and color-magnitude diagrams consistent with cluster membership. For the stars with well-calibrated photoelectric photometry, this means the star should not fall below the Pleiades single-star locus by more than about 0.2 mag or above that locus by more than about 1.0 mag (the expected displacement for a hierarchical triple with three nearly equal mass components). For stars with only photographic optical photometry, where the 1 uncertainties are of order 0.1–0.2 mag, we still require the star's photometry to be consistent with membership, but the allowed displacements from the single-star locus are considerably larger. Where accurate radial velocities are known, we require that the star be considered a radial velocity member based on the paper where the radial velocities were presented. Where stars have been previously identified as nonmembers based on photometric or spectroscopic indices, we adopt those conclusions. Two other relevant pieces of information are sometimes available. In some cases, individual proper-motion membership probabilities are provided by the various membership surveys. If no other information is available, and if the membership probability for a given candidate is less than 0.1, we exclude that star from our final catalog. However, often a star appears in several catalogs; if it appears in two or more proper-motion membership lists, we include it in the final catalog even if P < 0.1 in one of those catalogs. Second, an entirely different means to identify candidate Pleiades members is via flare star surveys toward the cluster (Haro et al. 1982; Jones 1981). A star with a formally low membership probability in one catalog but whose photometry is consistent with membership and that was identified as a flare star is retained in our catalog. Further details of the catalog construction are provided in the Appendix, as are details of the means by which the B, V, and I photometry have been homogenized. A full discussion and listing of all of the papers from which we have extracted astrometric and photometric information is also provided in the Appendix. Here we simply provide a very brief description of the inputs to the catalog. We include candidate cluster members from the following proper-motion surveys: Trumpler (1921), Hertzsprung (1947), Jones (1981), Pels & Lub (as reported in van Leeuwen et al. 1986), Stauffer et al. (1991), Artyukhina (1969), Hambly et al. (1993), Pinfield et al. (2000), Adams et al. (2001), and Deacon & Hambly (2004). Another important compilation that provides the initial identification of a significant number of low-mass cluster members is the flare star catalog of Haro et al. (1982). Table 1 provides a brief synopsis of the characteristics of the candidate member catalogs from these papers. The Trumpler paper is listed twice in Table 1 because there are two membership surveys included in that paper, with differing spatial coverages and different limiting magnitudes. Table 1 CITED IN TEXT | ASCII | TYPESET IMAGE Table 1 Pleiades Membership Surveys Used as Sources Go to: Table 2 Pleiades Membership Surveys Used as Sources Reference Area Covered (deg2) Magnitude Range (and Band) Number of Candidates Name Prefix Trumpler (1921)... 3 2.5 < B < 14.5 174 Tr Trumpler (1921)a... 24 2.5 < B < 10 72 Tr Hertzsprung (1947)... 4 2.5 < V < 15.5 247 HII Artyukhina (1969)... 60 2.5 < B < 12.5 Haro et al. (1982)... 20 11 < V < 17.5 519 HCG van Leeuwen et al. (1986)... 80 2.5 < B < 13 193 Pels Stauffer et al. (1991)... 16 14 < V < 18 225 SK Hambly et al. (1993)... 23 10 < I < 17.5 440 HHJ Pinfield et al. (2000)... 6 13.5 < I < 19.5 339 BPL Adams et al. (2001)... 300 8 < Ks < 14.5 1200 ... Deacon & Hambly (2004)... 75 10 < R < 19 916 DH a 200 AK The Trumpler paper is listed twice because there are two membership surveys included in that paper, with differing spatial coverages and different limiting magnitudes. In our final catalog, we have attempted to follow the standard naming convention whereby the primary name is derived from the paper where it was first identified as a cluster member. An exception to this arises for stars with both Trumpler (1921) and Hertzsprung (1947) names, where we use the Hertzsprung numbers as the standard name because that is the most commonly used designation for these stars in the literature. The failure for the Trumpler numbers to be given precedence in the literature perhaps stems from the fact that the Trumpler catalog was published in the Lick Observatory Bulletins as opposed to a refereed journal. In addition to providing a primary name for each star, we provide cross-identifications to some of the other catalogs, particularly where there is existing photometry or spectroscopy of that star using the alternate names. For the brightest cluster members, we provide additional cross-references (e.g., Greek names, Flamsteed numbers, HD numbers). For each star, we attempt to include an estimate for Johnson B and V, and for Cousins I (IC). Only a very small fraction of the cluster members have photoelectric photometry in these systems, unfortunately. Photometry for many of the stars has often been obtained in other systems, including Walraven, Geneva, Kron, and Johnson. We have used previously published transformations from the appropriate indices in those systems to Johnson BV or Cousins I. In other cases, photometry is available in a natural I-band system, primarily for some of the relatively faint cluster members. We have attempted to transform those I-band data to IC by deriving our own conversion using stars for which we already have an IC estimate as well as the natural I measurement. Details of these issues are provided in the Appendix. Finally, we have cross-correlated the cluster candidates catalog with the 2MASS All-Sky PSC and also with the 6x-PSC for the Pleiades. For every star in the catalog, we obtain JHKs photometry and 2MASS positions. Where we have both main survey 2MASS data and data from the 6x catalog, we adopt the 6x data for stars with J > 11, and data from the standard 2MASS catalog otherwise. We verified that the two catalogs do not have any obvious photometric or astrometric offsets relative to each other. The coordinates we list in our catalog are entirely from these 2MASS sources, and hence they inherit the very good and homogeneous 2MASS positional accuracies of order 0.1 rms. We have then plotted the candidate Pleiades members in a variety of color-magnitude diagrams and color-color diagrams and required that a star must have photometry that is consistent with cluster membership. Figure 8 illustrates this process and indicates why (for example) we have excluded HII 1695 from our final catalog. Fig. 8 Ks vs. Ks - [4.5] CMD for Pleiades candidate members, illustrating why we have excluded HII 1695 from the final catalog of cluster members. The "X" symbol marks the location of HII 1695 in this diagram. Table 2 provides the collected data for the 1417 stars we have retained as candidate Pleiades members. The first two columns are the J2000.0 right ascension and declination from 2MASS; the next are the 2MASS JHKs photometry and their uncertainties, and the 2MASS photometric quality flag ("ph-qual"). If the number following the 2MASS quality flag is a 1, the 2MASS data come from the 2MASS All-Sky PSC; if it is a 2, the data come from the 6x-PSC. The next three columns provide the B, V, and IC photometry, followed by a flag that indicates the provenance of that photometry. The last column provides the most commonly used names for these stars. The hydrogen-burning mass limit for the Pleiades occurs at about V = 22, I = 18, Ks = 14.4. Fifty-three of the candidate members in the catalog are fainter than this limit and hence should be substellar if they are indeed Pleiades members. R.A. (J2000.0) (deg) Dec. (J2000.0) (deg) 51.898273. .. Referenc e Name s 11.8 5 22 DH 001 10.3 0 ... 4 Pels 121 ... ... 14.3 4 22 DH 003 AAA 1 ... ... 15.0 5 22 DH 004 9.723 ± 0.016 AAA 1 12.5 9 11.7 5 ... 9 AKIII 59 12.44 2± 0.029 12.15 3± 0.015 AAA 1 ... ... 14.7 5 22 DH 006 10.42 9± 0.019 10.00 6± 0.029 9.924 ± 0.016 AAA 1 ... ... 11.1 6 22 DH 007 25.65230 4 8.459 ± 0.015 8.314 ± 0.059 8.270 ± 0.031 AAA 1 9.90 9.43 ... 9 AKIII 79 52.409874. .. 24.51054 6 10.31 8± 0.023 9.856 ± 0.028 9.698 ± 0.016 AAA 1 ... ... 11.1 9 22 DH 008 52.494766. .. 23.37185 9 12.79 9± 0.018 12.20 6± 0.019 11.94 6± 0.018 AAA 1 ... ... 14.3 9 22 DH 009 52.534420. .. 22.64416 3 13.68 0± 12.96 8± 12.77 0± AAA 1 ... ... 15.1 6 22 DH 010 J H Ks 24.52866 0 10.78 1± 0.025 10.06 6± 0.030 9.892 ± 0.017 51.925262. .. 23.80368 8 9.066 ± 0.013 8.754 ± 0.009 51.976067. .. 24.93647 8 12.88 0± 0.019 52.006481. .. 23.07849 9 52.168613. .. phquala B V IC AAA 1 ... ... 8.679 ± 0.014 AAA 1 10.9 6 12.21 9± 0.030 11.98 1± 0.016 AAA 1 13.52 5± 0.022 12.91 9± 0.022 12.61 9± 0.021 25.60778 2 10.19 8± 0.019 9.883 ± 0.029 52.200249. .. 25.57584 2 13.07 2± 0.019 52.203186. .. 26.49935 0 52.355843. .. Table 2 Pleiade s Members: Literature Photometr y 0.022 0.024 0.023 52.639614. .. 26.21576 7 11.00 7± 0.018 10.40 0± 0.027 10.27 9± 0.016 AAA 1 ... ... 11.8 6 22 DH 011 52.647411. .. 23.05233 4 15.31 9± 0.051 14.56 5± 0.060 14.26 0± 0.073 AAA 1 ... ... 16.6 8 22 DH 012 52.656086. .. 26.34610 0 14.23 9± 0.026 13.52 4± 0.037 13.29 9± 0.032 AAA 1 ... ... 15.7 3 22 DH 013 52.799591. .. 25.16510 0 11.48 9± 0.016 10.78 4± 0.020 10.60 2± 0.014 AAA 1 ... ... 12.6 5 22 DH 014 52.810955. .. 25.98114 8 11.99 1± 0.018 11.29 1± 0.022 11.09 1± 0.018 AAA 1 ... ... 13.3 0 22 DH 015 52.873249. .. 26.50352 5 13.61 3± 0.022 13.01 8± 0.026 12.75 8± 0.022 AAA 1 ... ... 15.2 8 22 DH 016 52.890076. .. 26.26550 7 9.514 ± 0.016 9.222 ± 0.021 9.068 ± 0.015 AAA 1 11.4 5 10.7 7 ... 4 Pels 008 53.001957. .. 23.77490 0 11.32 9± 0.017 10.68 6± 0.019 10.52 0± 0.016 AAA 1 15.2 7 13.9 5 ... 4 Pels 109 53.032749. .. 23.23265 5 13.13 5± 0.019 12.48 6± 0.019 12.25 4± 0.018 AAA 1 ... ... 14.7 1 22 DH 017 Note.— Table 2 is available in its entirety via the link to the machine-readable version above. a Standard 2MASS photometric data quality flag for JHKs, in that order. If the number following the 2MASS quality flags is a 1, the 2MASS data come from the standard 2MASS catalog; if it is a 2, the data come from the deep catalog. Table 3 provides the IRAC [3.6], [4.5], [5.8], and [8.0] photometry we have derived for Pleiades candidate members included within the region covered by the IRAC shallow survey of the Pleiades (see § 2). The brightest stars are saturated even in our short integration frame data, particularly for the more sensitive 3.6 and 4.5 m channels. At the faint end, we provide photometry only for 3.6 and 4.5 m because the objects are undetected in the two longer wavelength channels. At the "top" and "bottom" of the survey region, we have incomplete wavelength coverage for a band of width about 5 , and for stars in those areas we report only photometry in either the 3.6 and 5.8 bands or the 4.5 and 8.0 bands. Table 3 Pleiades Members: IRAC Photometry Name [3.6] [4.5] [5.8] [8] HHJ 107... 12.550 12.514 12.474 12.388 HCG 96... 11.869 11.881 11.805 11.824 DH 257... 9.604 9.608 9.604 9.554 SK 646... 11.318 11.273 11.204 11.215 HII 97... ... 9.760 ... 9.666 Pels 056... 9.188 9.214 9.164 9.165 HCG 112... 11.711 11.646 11.623 11.620 SK 622... 11.686 11.656 11.699 11.575 HCG 115... 11.450 11.434 11.316 11.437 HII 153... 7.163 7.205 7.183 7.198 HII 174... ... 9.325 ... 9.285 HII 173... 8.798 8.812 8.763 8.768 HCG 125... 11.641 11.594 11.564 11.540 Pels 043... 9.673 ... 9.673 ... SK 609... 15.572 15.663 15.857 15.685 AK 1B146... 8.189 8.175 8.153 8.166 HHJ 218... 12.309 12.285 12.268 12.494 HCG 126... 11.220 11.205 11.181 11.193 SK 596... 11.141 11.112 11.039 11.061 HCG 129... ... 11.276 ... 11.282 HCG 134... 11.111 11.071 11.032 11.033 HCG 131... 9.941 9.982 9.980 9.911 HII 250... ... 9.083 ... 9.023 Pels 059... 9.950 9.991 9.951 9.934 HHJ 235... 12.167 12.085 11.940 12.080 HCG 138... 11.200 11.132 11.096 11.124 HCG 143... 11.355 11.303 11.258 11.291 HHJ 100... 12.588 12.551 12.462 12.459 HCG 152... 10.858 10.874 10.857 10.846 HHJ 68... 13.007 12.914 13.089 12.688 HCG 157... 12.194 ... 12.045 ... HII 380... 10.169 10.192 10.173 10.161 HHJ 46... 13.149 13.085 13.170 12.898 Pels 041... 9.740 9.774 9.674 9.698 HII 430... 9.509 9.459 9.356 9.465 HHJ 24... 13.410 13.345 13.298 13.784 HCG 166... 12.168 12.111 11.908 12.291 HII 447... ... ... 5.522 5.528 HII 468... ... ... 4.010 3.910 HHJ 183... 12.458 12.426 12.431 12.207 HII 489... 8.857 8.885 8.864 8.814 DH 367... 12.770 12.717 12.594 12.589 HHJ 139... ... 12.495 ... 12.628 HII 514... 9.019 9.006 8.955 8.978 SK 534... 11.269 11.262 11.163 11.241 HII 531... 7.715 7.731 7.703 7.719 HHJ 164... 12.469 12.381 12.354 12.410 HII 554... ... 10.456 ... 10.427 HII 563... ... ... 4.620 4.580 HHJ 14... 13.479 13.461 13.624 13.430 HCG 180... 12.854 12.775 12.763 12.697 HII 559... ... 10.120 ... 10.097 HII 566... ... 10.722 ... 10.681 HII 571... 9.175 9.151 9.161 9.097 SK 526... ... 10.744 ... 10.698 HCG 181... ... 11.173 ... 11.141 HCG 178... 10.938 10.960 10.774 10.767 HII 590... ... 10.552 ... 10.503 HII 625... 9.317 9.330 9.323 9.250 HHJ 273... ... 12.014 ... 11.962 DH 392... 11.972 11.934 11.827 12.053 HII 652... 7.426 7.455 7.422 7.278 HHJ 99... 13.052 13.021 13.180 13.126 HHJ 106... 12.975 12.923 12.953 12.619 HII 676... 10.149 10.131 10.105 10.132 HII 673... 10.938 10.944 10.910 10.997 HHJ -293... 12.227 ... 12.060 ... HII 686... 10.100 10.116 10.126 10.084 HII 697... 7.703 7.661 7.656 7.597 HII 708... 8.547 8.507 8.472 8.497 HII 717... 6.538 6.600 6.592 6.611 HCG 196... 10.767 10.810 10.767 10.749 HHJ 130... 12.724 12.695 12.618 12.781 HII 738... 8.819 8.845 8.773 8.762 HII 745... 8.002 8.007 7.975 7.988 HCG 195... 10.648 10.611 10.561 10.587 HII 746... 9.300 9.328 9.292 9.280 HII 740... ... 10.495 ... 10.425 HII 762... 10.556 10.571 10.533 10.479 HII 761... 8.736 8.745 8.722 8.694 HII 793... 10.529 10.621 10.493 10.431 DH 403... 14.486 14.432 14.604 13.951 BPL 77... 11.528 11.500 11.390 55.505 HII 785... ... ... 4.050 4.030 HII 799... 10.140 10.162 10.107 10.178 BPL 79... 14.069 14.009 13.914 13.960 HII 804... 7.345 7.323 7.340 7.338 HHJ 166... 12.469 12.405 12.364 12.410 BPL 81... 14.498 14.477 14.190 14.098 HII 813... 10.355 10.340 10.283 10.288 HII 817... ... 9.900 ... 5.737 BPL 82... 11.539 11.510 11.449 11.582 SK 497... 11.198 11.172 11.112 11.107 HHJ 27... 13.407 13.418 13.113 13.243 DH 412... 13.309 13.297 13.161 13.409 HHJ 127... 12.785 12.736 12.774 12.486 SK 491... 11.042 11.052 10.994 11.016 HII 870... 9.133 9.134 9.109 9.082 HII 859... ... 6.454 ... 6.422 10.656 10.684 10.635 SK 490... 10.672 HHJ 363... 11.628 11.536 11.505 11.552 BPL 88... 12.871 12.811 12.703 12.560 SK 488... ... 11.196 ... 11.137 HHJ 194... 12.575 12.635 12.546 12.712 HII 879... ... 10.081 ... 10.066 HHJ 435... 10.767 10.761 10.733 10.702 HII 883... ... 10.195 ... 10.125 HII 890... 10.727 10.715 10.649 10.696 HII 916... ... 9.524 ... 9.479 HII 930... 10.459 10.472 10.424 10.459 HHJ 56... 12.475 12.422 12.572 12.562 HII 956... 7.092 7.131 7.079 7.069 HII 980... ... ... ... 4.190 HHJ 105... ... 12.724 ... 12.609 DH 441... 11.766 11.693 11.531 11.636 HII 996... ... 8.932 ... 8.878 HCG 218... 12.534 12.462 12.443 12.465 HHJ 249... 12.259 12.254 12.173 12.154 HCG 219... 10.889 10.851 10.794 10.801 HHJ 326... 11.786 11.731 11.618 11.719 HII 1028... 7.078 7.120 7.113 7.085 HII 1015... ... 9.016 ... 8.965 HHJ 161... 12.285 12.181 12.275 12.181 HII 1039... 9.798 ... 9.764 ... HII 1032... 9.143 9.129 9.144 9.071 HII 1061... 10.298 10.323 10.245 10.240 HII 1084... 7.052 ... 7.043 ... HHJ 140... 12.439 12.385 12.431 12.409 HII 1094... 10.549 10.546 10.479 10.613 HII 1100... 9.285 9.321 9.302 9.264 HII 1117... 8.497 8.524 8.543 8.482 HII 1110... 10.227 10.284 10.240 10.208 HII 1122... 8.149 8.146 8.174 8.132 HII 1124... 9.858 9.843 9.847 9.785 HHJ 104... 12.705 ... 12.752 ... DH 467... 11.379 11.297 11.253 11.271 HII 1173... ... 10.855 ... 10.781 HHJ 247... 11.836 ... 11.660 ... HCG 244... 10.955 10.921 10.873 10.870 HII 1215... 8.997 ... 8.953 ... HHJ 257... 11.948 11.924 11.906 11.714 HHJ 174... 12.506 ... ... ... HHJ 299... 11.797 11.744 11.694 11.663 HII 1234... 6.729 6.743 6.712 6.679 HHJ 252... 11.927 11.863 11.740 11.836 HII 1280... 10.548 10.602 10.528 10.575 HII 1286... 10.378 ... 10.289 ... HII 1284... 7.617 7.627 7.571 7.589 HII 1298... 9.778 9.784 9.800 9.741 HCG 253... 11.625 11.513 11.511 11.559 HII 1306... 9.798 9.811 9.773 9.747 HHJ 37... 13.241 13.158 13.051 13.073 HII 1321... 10.438 10.413 10.361 10.372 HII 1309... 8.270 8.285 8.244 8.259 HHJ 92... 12.816 12.717 12.684 12.753 HII 1332... 9.969 10.016 9.991 9.958 HCG 258... 10.839 10.766 10.673 10.750 HII 1338... 7.463 7.483 7.503 7.476 HII 1348... 9.622 9.651 9.622 9.575 HII 1355... 10.016 10.024 9.991 10.035 HII 1362... 7.637 7.661 7.641 7.618 HII 1380... 6.914 6.952 6.961 6.962 HII 1375... ... 6.323 6.311 6.318 HCG 266... 12.175 12.115 12.078 11.993 HII 1384... ... 6.984 ... 6.979 HII 1397... 7.181 7.196 7.192 7.184 HHJ 198... 12.717 12.668 12.541 12.431 HCG 269... 11.965 ... 11.859 ... HII 1425... 7.342 ... 7.323 ... HII 1431... 6.640 6.652 6.644 6.674 HCG 273... 11.175 11.142 11.093 11.240 HCG 277... 10.658 ... 10.553 ... HII 1454... ... 10.071 ... 10.026 DH 523... 12.495 12.493 12.472 12.348 HII 1516... 10.219 10.252 10.211 10.217 HII 1514... 8.940 8.936 8.910 8.913 HII 1532... 10.510 10.472 10.450 10.403 HII 1531... 10.260 10.275 10.191 10.195 HHJ 26... 13.951 13.840 13.870 13.714 HHJ 152... 12.384 ... 12.228 ... HHJ 438... 11.138 11.076 11.014 11.048 HCG 295... 11.248 11.254 11.248 11.190 HHJ 122... 12.698 ... 12.635 ... HII 1613... 8.562 8.565 8.514 8.535 HHJ 240... 12.232 12.184 12.131 12.102 HII 1726... 7.905 7.904 7.883 7.891 HCG 307... 11.929 11.899 11.830 11.887 HHJ 156... 12.458 ... 12.384 ... HHJ 225... 12.370 12.237 12.267 12.356 DH 555... 13.164 13.108 12.971 13.066 HCG 311... 12.095 12.014 11.969 12.067 HII 1762... 7.306 7.312 7.342 7.332 HCG 315... 11.888 ... 11.717 ... HHJ 336... 11.528 ... 11.462 ... HII 1797... 8.729 ... 8.662 ... HII 1794... 8.871 8.887 8.812 8.797 HII 1785... 10.569 10.594 10.552 10.563 HHJ 188... 12.467 12.384 12.441 12.456 HII 1827... 10.298 10.272 10.163 10.217 HII 1856... 8.648 8.637 8.613 8.626 HCG 328... 12.677 12.611 12.659 12.887 HII 1876... 6.575 6.578 6.595 6.592 HCG 324... 11.169 11.146 11.114 11.146 HCG 327... 12.521 12.483 12.455 12.506 HHJ 184... 12.586 12.555 12.573 12.553 HII 1912... 7.798 7.834 7.783 7.790 HCG 335... 12.866 12.826 12.760 12.820 HCG 337... 11.627 11.558 11.584 11.665 HHJ 44... 13.179 13.093 13.023 12.903 HHJ 207... 12.389 12.304 12.221 12.330 HII 2027... 8.788 8.836 8.774 8.784 HII 2034... 9.878 9.922 9.923 9.865 DH 593... 14.247 14.270 14.972 14.808 HHJ 8... 13.674 13.656 13.473 13.729 HHJ 231... 12.236 12.194 12.171 12.189 HCG 354... 10.566 ... 10.486 ... HII 2147... 8.558 8.615 8.514 8.549 HII 2168... ... ... 3.840 3.820 HII 2195... 7.600 7.622 7.601 7.588 DH 610... 13.173 ... 13.132 ... HII 2284... 9.366 9.384 9.353 9.319 HII 2311... 9.445 ... 9.395 ... HHJ 142... 12.426 ... 12.224 ... Because Table 2 is an amalgam of many previous catalogs, each of which have different spatial coverage, magnitude limits, and other idiosyncrasies, it is necessarily incomplete and inhomogeneous. It also certainly includes some nonmembers. For V < 12, we expect very few nonmembers because of the extensive spectroscopic data available for those stars; the fraction of nonmembers will likely increase to fainter magnitudes, particularly for stars located far from the cluster center. The catalog is simply an attempt to collect all of the available data, identify some of the nonmembers, and eliminate duplications. We hope that it will also serve as a starting point for future efforts to produce a "cleaner" catalog. Figure 9 shows the distribution on the sky of the stars in Table 2. The complete spatial distribution of all members of the Pleiades may differ slightly from what is shown due to the inhomogeneous properties of the proper-motion surveys. However, we believe that those effects are relatively small and the distribution shown is mostly representative of the parent population. One thing that is evident in Figure 9 is mass segregation—the highest mass cluster members are much more centrally located than the lowest mass cluster members. This fact is reinforced by calculating the cumulative number of stars as a function of distance from the cluster center for different absolute magnitude bins. Figure 10 illustrates this fact. Another property of the Pleiades illustrated by Figure 10 is that the cluster appears to be elongated parallel to the Galactic plane, as expected from n-body simulations of galactic clusters (Terlevich 1987). Similar plots showing the flattening of the cluster and evidence for mass segregation for the V < 12 cluster members were provided by Raboud & Mermilliod (1998). Fig. 9 Spatial plot of the candidate Pleiades members from Table 2. The large star symbols are members brighter than Ks = 6; the open circles are stars with 6 < Ks < 9; and the dots are candidate members fainter than Ks = 9. The solid line is parallel to the Galactic plane. Fig. 10 Cumul ative radial density profiles for Pleiades members in several magnitude ranges: heavy, longdashed line, Ks < 6; dots, 6 < Ks < 9; shortdashed line, 9 < Ks < 12; light, longdashed line, Ks > 12. 4. EMPIRICAL PLEIADES ISOCHRONES AND COMPARISON TO MODEL ISOCHRONES Young, nearby, rich open clusters like the Pleiades can and should be used to provide template data that can help interpret observations of more distant clusters or to test theoretical models. The identification of candidate members of distant open clusters is often based on plots of stars in a color-magnitude diagram, overlaid on which is a line meant to define the single-star locus at the distance of the cluster. The stars lying near or slightly above the locus are chosen as possible or probable cluster members. The data we have collected for the Pleiades provide a means to define the single-star locus for 100 Myr, solar metallicity stars in a variety of widely used color systems down to and slightly below the hydrogen-burning mass limit. Figures 11 and 12 illustrate the appearance of the Pleiades stars in two of these diagrams, and the single-star locus we have defined. The curve defining the single-star locus was drawn entirely "by eye." It is displaced slightly above the lower envelope to the locus of stars to account for photometric uncertainties (which increase to fainter magnitudes). We attempted to use all of the information available to us, however. That is, there should also be an upper envelope to the Pleiades locus in these diagrams, since equal-mass binaries should be displaced above the single-star sequence by 0.7 mag (and one expects very few systems of higher multiplicity). Therefore, the single-star locus was defined with that upper envelope in mind. Table 4 provides the single-star loci for the Pleiades for BVICJKs plus the four IRAC channels. We have dereddened the empirical loci by the canonical mean extinction to the Pleiades of AV = 0.12 (and, correspondingly, AB = 0.16, AI = 0.07, AJ = 0.03, and AK = 0.01, as per the reddening law of Rieke & Lebofsky 1985). Fig. 11 V vs. (V - I)c CMD for Pleiades members with photoelectric photometry. The solid curve is the "by-eye" fit to the single-star locus for Pleiades members. Fig. 12 Ks vs. Ks - [3.6] CMD for Pleiades candidate members from Table 2 (dots) and from deeper imaging of a set of Pleiades VLM and brown dwarf candidate members from P. Lowrance et al. (2007, in preparation) (squares). The solid curve is the single-star locus from Table 4. (51 kB) B V IC Ks [3.6] [4.5] [5.8] [8] 6.598... 6.600 6.574 6.592 6.602 6.615 6.602 6.602 6.706... 6.700 6.665 6.671 6.682 6.695 6.682 6.682 6.814... 6.800 6.755 6.750 6.761 6.775 6.762 6.762 6.922... 6.900 6.848 6.834 6.841 6.855 6.841 6.841 7.030... 7.000 6.940 6.910 6.920 6.935 6.921 6.921 7.142... 7.100 7.030 6.982 6.990 7.005 6.991 6.991 7.254... 7.200 7.115 7.039 7.050 7.064 7.050 7.049 7.370... 7.300 7.200 7.104 7.109 7.124 7.108 7.107 7.490... 7.400 7.283 7.162 7.168 7.183 7.165 7.164 7.610... 7.500 7.367 7.228 7.238 7.252 7.233 7.231 7.730... 7.600 7.450 7.287 7.297 7.311 7.291 7.288 7.850... 7.700 7.533 7.345 7.347 7.360 7.339 7.336 7.968... 7.800 7.615 7.387 7.396 7.409 7.387 7.384 8.084... 7.900 7.698 7.428 7.436 7.449 7.426 7.422 8.200... 8.000 7.780 7.469 7.475 7.488 7.465 7.460 Table 4 Single-Star Pleiades Loci 8.320... 8.100 7.840 7.508 7.515 7.527 7.503 7.498 8.440... 8.200 7.900 7.546 7.555 7.567 7.542 7.536 8.564... 8.300 7.975 7.591 7.594 7.606 7.580 7.575 8.692... 8.400 8.050 7.648 7.654 7.665 7.638 7.632 8.820... 8.500 8.125 7.701 7.703 7.715 7.687 7.680 8.936... 8.600 8.200 7.762 7.762 7.774 7.744 7.737 The other benefit to constructing the new catalog is that it can provide an improved comparison data set to test theoretical isochrones. The new catalog provides homogeneous photometry in many photometric bands for stars ranging from several solar masses down to below 0.1 M . We take the distance to the Pleiades as 133 pc and refer the reader to Soderblom et al. (2005) for a discussion and a listing of the most recent determinations. The age of the Pleiades is not as well-defined but is probably somewhere between 100 and 125 Myr (Meynet et al. 1993; Stauffer et al. 1999). We adopt 100 Myr for the purposes of this discussion; our conclusions relative to the theoretical isochrones would not be affected significantly if we instead chose 125 Myr. As noted above, we adopt AV = 0.12 as the mean Pleiades extinction and apply that value to the theoretical isochrones. A small number of Pleiades members have significantly larger extinctions (Breger 1986; Stauffer & Hartmann 1987), and we have dereddened those stars individually to the mean cluster reddening. Figures 13 and 14 compare theoretical 100 Myr isochrones from Siess et al. (2000) and Baraffe et al. (1998) to the Pleiades member photometry from Table 2 for stars for which we have photoelectric photometry. Neither set of isochrones are a good fit to the V - I based color-magnitude diagram. For Baraffe et al. (1998) this is not a surprise because they illustrated that their isochrones are too blue in V - I for cool stars in their paper and ascribed the problem as likely the result of an incomplete line list, resulting in too little absorption in the V band. For Siess et al. (2000) the poor fit in the V - I CMD is somewhat unexpected in that they transform from the theoretical to the observational plane using empirical colortemperature relations. In any event, it is clear that neither model isochrones match the shape of the Pleiades locus in the V versus V - I plane, and therefore use of these V - I based isochrones for younger clusters is not likely to yield accurate results (unless the color-Teff relation is recalibrated, as described, e.g., in Jeffries & Oliveira 2005). On the other hand, the Baraffe et al. (1998) model provides a quite good fit to the Pleiades single-star locus for an age of 100 Myr in the K versus I - K plane.6 This perhaps lends support to the hypothesis that the misfit in the V versus V - I plane is due to missing opacity in their V-band atmospheres for low-mass stars (see also Chabrier et al. 2000 for further evidence in support of this idea). The Siess et al. (2000) isochrones do not fit the Pleiades locus in the K versus I - K plane particularly well, being too faint near I - K = 2 and too bright for I - K > 2.5. Fig. 13 V vs. (V I)c CMD for Pleiades candidate members from Table 2 for which we have photoelectric photometry, compared to theoretical isochrones from Siess et al. (2000) (left) and from Baraffe et al. (1998) (right). For the left panel, the curves correspond to 10, 50, and 100 Myr and a ZAMS; the right panel includes curves for 50 and 100 Myr and a ZAMS. (74 kB) Fig. 14 K vs. (I - K) CMD for Pleiades candidate members from Table 2, compared to theoretical isochrones from Siess et al. (2000) (left) and from Baraffe et al. (1998) (right). The curves correspond to 50 and 100 Myr and a ZAMS. 6 These isochrones are calculated for the standard K filter, rather than Ks. However, the difference in location of the isochrones in these plots because of this should be very slight, and we do not believe our conclusions are significantly affected. 5. IDENTIFICATION OF NEW VERY LOW-MASS CANDIDATE MEMBERS The highest spatial density for Pleiades members of any mass should be at the cluster center. However, searches for substellar members of the Pleiades have generally avoided the cluster center because of the deleterious effects of scattered light from the high-mass cluster members and because of the variable background from the Pleiades reflection nebulae. The deep 2MASS and IRAC 3.6 m imaging and 4.5 m imaging provide accurate photometry to well below the hydrogen-burning mass limit and are less affected by the nebular emission than shorter wavelength images. We therefore expect that it should be possible to identify a new set of candidate Pleiades substellar members by combining our new near- and mid-infrared photometry. The substellar mass limit in the Pleiades occurs at about Ks = 14.4, near the limit of the 2MASS All-Sky PSC. As illustrated in Figure 15, the deep 2MASS survey of the Pleiades should easily detect objects at least 2 mag fainter than the substellar limit. The key to actually identifying those objects and separating them from the background sources is to find color-magnitude or color-color diagrams that separate the Pleiades members from the other objects. As shown in Figure 15, late-type Pleiades members separate fairly well from most field stars toward the Pleiades in a Ks versus Ks - [3.6] colormagnitude diagram. However, as illustrated in Figure 2, in the Ks magnitude range of interest there is also a large population of red galaxies, and they are in fact the primary contaminants to identifying Pleiades substellar objects in the Ks versus Ks - [3.6] plane. Fortunately, most of the contaminant galaxies are slightly resolved in the 2MASS and IRAC imaging, and we have found that we can eliminate most of the red galaxies by their nonstellar image shape. Figure 15 shows the first step in our process of identifying new very low-mass members of the Pleiades. The red plus symbols are the known Pleiades members from Table 2. The red open circles are candidate Pleiades substellar members from deep imaging surveys published in the literature, mostly of parts of the cluster exterior to the central square degree, where the IRAC photometry is from P. Lowrance et al. (2007, in preparation). The blue, filled circles are field M and L dwarfs, placed at the distance of the Pleiades, using photometry from Patten et al. (2006). Because the Pleiades is 100 Myr, its very low-mass stellar and substellar objects will be displaced about 0.7 mag above the locus of the field M and L dwarfs according to the Baraffe et al. (1998) and Chabrier et al. (2000) models, in accord with the location in the diagram of the previously identified, candidate VLM and substellar objects. The trapezoidal shaped region outlined with a dashed line is the region in the diagram that we define as containing candidate new VLM and substellar members of the Pleiades. We place the faint limit of this region at Ks = 16.2 in order to avoid the large apparent increase in faint, red objects for Ks > 16.2, caused largely by increasing errors in the Ks photometry. Also, the 2MASS extended object flags cease to be useful fainter than about Ks = 16. Fig. 15 Ks vs. Ks - [3.6] CMD for the objects in the central 1 deg2 of the Pleiades, combining data from the IRAC shallow survey and 2MASS. The symbols are defined within the figure (and see text for details). The dashedline box indicates the region within which we have searched for new candidate Pleiades VLM and substellar members. The solid curve is a DUSTY 100 Myr isochrone from Chabrier et al. (2000) for masses from 0.1 to 0.03 M . We took the following steps to identify a set of candidate substellar members of the Pleiades: keep only objects that fall in the trapezoidal region in Figure 15; remove objects flagged as nonstellar by the 2MASS pipeline software; remove objects that appear nonstellar to the eye in the IRAC images; remove objects that do not fall in or near the locus of field M and L dwarfs in a J - H versus H - Ks diagram; remove objects that have 3.6 and 4.5 m magnitudes that differ by more than 0.2 mag; remove objects that fall below the ZAMS in a J versus J - Ks diagram. As shown in Figure 15, all stars earlier than about mid-M have Ks - [3.6] colors bluer than 0.4. This ensures that for most of the area of the trapezoidal region, the primary contaminants are distant galaxies. Fortunately, the 2MASS catalog provides two types of flags for identifying extended objects. For each filter, a 2 flag measures the match between the objects shape and the instrumental PSF, with values greater than 2.0 generally indicative of a nonstellar object. In order not to be misguided by an image artifact in one filter, we throw out the most discrepant of the three flags and average the other two. We discard objects with mean 2 greater than 1.9. The other indicator is the 2MASS extended object flag, which is the synthesis of several independent tests of the objects shape, surface brightness and color (see Jarrett et al. 2000 for a description of this process). If one simply excludes the objects classified as extended in the 2MASS 6x image by either of these techniques, the number of candidate VLM and substellar objects lying inside the trapezoidal region decreases by nearly a half. We have one additional means to demonstrate that many of the identified objects are probably Pleiades members, and that is via proper motions. The mean Pleiades proper motion is R.A. = 20 mas yr-1 and decl. = -45 mas yr-1 (Jones 1973). With an epoch difference of only 3.5 yr between the deep 2MASS and IRAC imaging, the expected motion for a Pleiades member is only 0.07 in right ascension and -0.16 in declination. Given the relatively large pixel size for the two cameras, and the undersampled nature of the IRAC 3.6 and 4.5 m images, it is not a priori obvious that one would expect to reliably detect the Pleiades motion. However, both the 2MASS and IRAC astrometric solutions have been very accurately calibrated. Also, for the present purpose, we only ask whether the data support a conclusion that most of the identified substellar candidates are true Pleiades members (i.e., as an ensemble), rather than that each star is well enough separated in a VPD to derive a high membership probability. Figure 16 provides a set of plots that we believe support the conclusion that the majority of the surviving VLM and substellar candidates are Pleiades members. The first plot shows the measured motions between the epoch of the 2MASS and IRAC observations for all known Pleiades members from Table 2 that lie in the central square degree region and have 11 < Ks < 14 (i.e., just brighter than the substellar candidates). The mean offset of the Pleiades stellar members from the background population is well-defined and is quantitatively of the expected magnitude and sign (+0.07 in right ascension and -0.16 in declination). The rms dispersion of the coordinate difference for the field population in right ascension and declination is 0.076 and 0.062 , supportive of our claim that the relative astrometry for the two cameras is quite good. Because we expect that the background population should have essentially no mean proper motion, the nonzero mean "motion" of the field population of about R.A. = 0.3 is presumably not real. Instead, the offset is probably due to the uncertainty in transferring the Spitzer coordinate zero point between the warm star-tracker and the cryogenic focal plane. Because it is simply a zero-point offset applicable to all the objects in the IRAC catalog, it has no effect on the ability to separate Pleiades members from the field star population. Fig. 16 Proper-motion vector point diagrams (VPDs) for various stellar samples in the central 1° field, derived from combining the IRAC and 2MASS 6x observations. Top left: VPD comparing all objects in the field (small black dots) to Pleiades members with 11 < Ks < 14 (large blue dots). Top right: Same, except the blue dots are the new candidate VLM and substellar Pleiades members. Bottom left: Same, except the blue dots are a nearby, low-mass field star sample from a box just blueward of the trapezoidal region in 15. Bottom right: VPD just showing a second, distant field star sample as described in the text. The second panel in Figure 16 shows the proper motion of the candidate Pleiades VLM and substellar objects. While these objects do not show as clean a distribution as the known members, their mean motion is clearly in the same direction. After removing 2 deviants, the median offsets for the substellar candidates are 0.04 and -0.11 in right ascension and declination, respectively. The objects whose motions differ significantly from the Pleiades mean may be nonmembers or they may be members with poorly determined motions (since a few of the high-probability members in the first panel also show discrepant motions). The other two panels in Figure 16 show the proper motions of two possible control samples. The first control sample was defined as the set of stars that fall up to 0.3 mag below the lower sloping boundary of the trapezoid in Figure 15. These objects should be late-type dwarfs that are either older or more distant than the Pleiades or red galaxies. We used the 2MASS data to remove extended or blended objects from the sample in the same way as for the Pleiades candidates. If the objects are nearby field stars, we expect to see large proper motions; if galaxies, the real proper motions would be small—but relatively large apparent proper motions due to poor centroiding or different centroids at different effective wavelengths could be present. The second control set was defined to have -0.1 < K - [3.6] < 0.1 and 14.0 < K < 14.5 and to be stellar based on the 2MASS flags. This control sample should therefore be relatively distant G and K dwarfs primarily. Both control samples have proper-motion distributions that differ greatly from the Pleiades samples and that make sense for, respectively, a nearby and a distant field star sample. Figure 17 shows the Pleiades members from Table 2 and the 55 candidate VLM and substellar members that survived all of our culling steps. We cross-correlated this list with the stars from Table 2 and with a list of the previously identified candidate substellar members of the cluster from other deep imaging surveys. Fourteen of the surviving objects correspond to previously identified Pleiades VLM and substellar candidates. We provide the new list of candidate members in Table 5. The columns marked as (R.A.) and (decl.) are the measured motions in arcsec over the 3.5 yr epoch difference between the 2MASS-6x and IRAC observations. Forty-two of these objects have Ks > 14.0 and hence inferred masses less than about 0.1 M ; 31 of them have Ks > 14.4 and hence have inferred masses below the hydrogen-burning mass limit. (130 kB) Fig. 17 Same as Fig. 15, except that the new candidate VLM and substellar objects from Table 5 are now indicated as small, red squares. Table Candidate Members 5 New Pleiades R.A. (J2000.0) (deg) Decl. (J2000.0) (deg) J H Ks [3.6] [4.5] (R.A.) (decl.) Previous ID SI2M-1... 56.15745 24.42746 14.44 13.79 13.52 13.17 13.10 0.37 -0.01 HHJ 46 SI2M-2... 56.19235 24.38414 14.68 14.10 13.79 13.42 13.36 0.44 -0.19 HHJ 24 SI2M-3... 56.24477 24.27201 17.85 16.83 16.00 15.15 15.15 0.37 0.13 ... SI2M-4... 56.28952 23.97910 15.46 14.83 14.41 14.05 14.05 0.54 -0.16 ... SI2M-5... 56.29098 24.07576 14.80 14.16 13.86 13.43 13.37 0.45 -0.17 ... SI2M-6... 56.30265 23.89584 14.83 14.21 13.88 13.49 13.47 0.37 -0.14 HHJ 14 SI2M-7... 56.32663 23.87112 15.96 15.15 14.79 14.38 14.34 0.18 -0.01 ... SI2M-8... 56.36751 24.52373 16.84 16.05 15.44 ... 14.79 0.32 -0.05 ... SI2M-9... 56.39588 23.85472 15.78 15.02 14.65 14.27 14.18 0.37 0.08 ... SI2M-10... 56.40739 23.73057 14.79 14.15 13.81 13.37 13.40 0.33 -0.23 ... SI2M-11... 56.42205 23.90273 15.39 14.73 14.28 13.86 13.85 0.41 -0.10 ... SI2M-12... 56.42644 24.06976 15.27 14.64 14.28 13.89 13.95 0.36 -0.18 ... SI2M-13... 56.43118 23.64760 15.17 14.43 14.14 13.78 13.76 0.36 -0.22 ... SI2M-14... 56.44669 24.51118 17.25 16.37 15.75 ... 15.04 0.43 -0.25 ... SI2M-15... 56.45366 23.64644 17.53 16.49 15.57 14.91 14.62 0.21 -0.04 ... SI2M-16... 56.45598 23.95163 14.70 14.07 13.83 13.48 13.36 0.37 -0.08 ... SI2M-17... 56.45634 24.26979 18.11 16.71 16.18 15.38 15.21 0.76 0.10 ... SI2M-18... 56.46099 23.74362 16.39 15.70 15.28 14.64 14.79 0.34 0.01 ... SI2M-19... 56.46113 24.15099 15.81 15.06 14.64 14.08 14.02 0.30 -0.20 BPL 79 SI2M-20... 56.46912 23.86272 15.32 14.64 14.31 13.90 13.78 0.47 -0.18 ... SI2M-21... 56.47910 23.56604 15.57 14.96 14.55 14.18 ... 0.44 -0.34 ... SI2M-22... 56.49051 24.05142 14.72 14.12 13.79 13.43 13.44 0.37 -0.11 HHJ 27 SI2M-23... 56.49128 24.41130 16.74 16.09 15.54 14.88 14.82 1.17 0.10 ... SI2M-24... 56.49132 24.14474 14.58 13.99 13.68 13.32 13.32 0.22 -0.06 DH 412 ID SI2M-25... 56.52133 23.75971 15.56 14.81 14.38 13.98 13.93 0.29 -0.08 ... SI2M-26... 56.52526 23.97200 14.88 14.22 13.93 13.58 13.53 0.26 -0.14 ... SI2M-27... 56.57735 23.98407 14.75 14.11 13.86 13.49 13.39 0.30 -0.08 ... SI2M-28... 56.57843 23.81347 15.83 15.02 14.57 14.06 14.18 0.29 -0.18 ... SI2M-29... 56.58151 23.56235 15.80 15.08 14.69 14.30 ... 0.23 -0.14 ... SI2M-30... 56.58557 24.28870 17.05 16.31 15.76 15.09 14.87 -0.10 -0.12 ... SI2M-31... 56.59283 23.87408 15.59 14.94 14.46 14.10 14.01 0.45 -0.11 ... SI2M-32... 56.60060 24.50354 14.75 14.14 13.84 13.48 13.42 0.38 -0.22 BPL 101 SI2M-33... 56.60880 24.08598 15.19 14.52 14.15 13.72 13.74 0.40 -0.07 ... SI2M-34... 56.63392 24.38740 17.25 16.34 15.77 15.13 15.11 0.27 -0.23 ... SI2M-35... 56.64737 23.95206 15.37 14.77 14.45 14.06 13.97 0.29 -0.22 ... SI2M-36... 56.67914 24.41405 15.55 14.85 14.42 13.98 14.00 0.26 -0.17 BPL 108 SI2M-37... 56.70850 24.00659 15.69 14.98 14.58 14.05 14.00 0.34 -0.04 ... SI2M-38... 56.75776 24.22451 14.97 14.41 14.10 13.73 13.62 0.29 -0.06 BPL 122 SI2M-39... 56.77373 24.66767 15.47 14.87 14.51 ... 14.01 0.36 -0.07 ... SI2M-40... 56.79400 23.90606 15.99 15.28 15.02 14.57 14.54 0.19 0.01 ... SI2M-41... 56.79446 23.97119 14.95 14.38 14.02 13.66 13.66 0.32 -0.20 ... SI2M-42... 56.79918 24.22539 14.86 14.23 13.88 13.49 13.38 0.27 -0.08 BPL 130 SI2M-43... 56.80051 24.47547 16.19 15.53 15.05 14.57 14.65 0.41 -0.41 BPL 132 SI2M-44... 56.82203 24.20922 17.54 17.00 15.94 15.23 14.62 -0.05 -0.10 ... SI2M-45... 56.96009 23.91330 16.41 15.71 15.20 14.51 14.53 0.34 -0.16 ... SI2M-46... 56.96365 23.73669 17.52 16.67 16.02 15.22 15.11 0.25 -0.04 ... SI2M-47... 57.00899 24.42107 16.75 16.05 15.42 14.85 14.76 0.23 -0.08 ... SI2M-48... 57.01952 23.65838 15.28 14.66 14.27 13.78 ... 0.26 -0.15 ... SI2M-49... 57.07928 24.42024 16.02 15.27 14.95 14.49 14.51 0.36 -0.11 BPL 172 SI2M-50... 57.09851 24.37646 14.92 14.34 13.94 13.58 13.55 0.30 -0.01 BPL 177 SI2M-51... 57.12811 23.70665 16.66 15.85 15.19 14.57 ... 0.32 -0.12 ... SI2M-52... 57.13138 24.57707 16.78 15.88 15.38 14.70 14.65 0.18 -0.02 ... SI2M-53... 57.14174 24.08293 15.38 14.67 14.47 14.10 14.00 0.44 -0.16 ... SI2M-54... 57.23196 24.36115 15.04 14.43 14.10 13.69 13.66 0.36 -0.25 HHJ 8 SI2M-55... 57.28922 23.94612 17.70 16.77 16.09 15.14 15.02 0.54 0.27 ... Our candidate list could be contaminated by foreground late-type dwarfs that happen to lie in the line of sight to the Pleiades. How many such objects should we expect? In order to pass our culling steps, such stars would have to be mid- to late-M dwarfs, or early to mid-L dwarfs. We use the known M dwarfs within 8 pc to estimate how many field M dwarfs should lie in a 1 deg2 region and at distance between 70 and 100 pc (so they would be coincident in a CMD with the 100 Myr Pleiades members). The result is 3 such field M dwarf contaminants. Cruz et al. (2007) estimate that the volume density of L dwarfs is comparable to that for late-M dwarfs, and therefore a very conservative estimate is that there might also be 3 field L dwarfs contaminating our sample. We regard this (6 contaminating field dwarfs) as an upper limit because our various selection criteria would exclude early-M dwarfs and late-L dwarfs. Bihain et al. (2006) made an estimate of the number of contaminating field dwarfs in their Pleiades survey of 1.8 deg2; for the spectral type range of our objects, their algorithm would have predicted just one or two contaminating field dwarfs for our survey. How many substellar Pleiades members should there be in the region we have surveyed? That is, of course, part of the question we are trying to answer. However, previous studies have estimated that the Pleiades stellar mass function for M < 0.5 M can be approximated as a power law with an exponent of -1 (dN/dM M-1). Using the known Pleiades members from Table 2 that lie within the region of the IRAC survey and that have masses of 0.2 < M/M < 0.5 (as estimated from the Baraffe et al. (1998) 100 Myr isochrone) to normalize the relation, the M-1 mass function predicts about 48 members in our search region and with 14 < K < 16.2 (corresponding to 0.1 < M/M < 0.035). Other studies have suggested that the mass function in the Pleiades becomes shallower below 0.1 M , dN/dM M-0.6. Using the same normalization as above, this functional form for the Pleiades mass function for M < 0.1 M yields a prediction of 20 VLM and substellar members in our survey. The number of candidates we have found falls between these two estimates. Better proper motions and low-resolution spectroscopy will almost certainly eliminate some of these candidates as nonmembers. 6. MID-IR OBSERVATIONS OF DUST AND POLYCYCLIC AROMATIC HYDROCARBONS IN THE PLEIADES Since the earliest days of astrophotography, it has been clear that the Pleiades stars are in relatively close proximity to interstellar matter whose optical manifestation is the spider-web–like network of filaments seen particularly strongly toward several of the B stars in the cluster. High-resolution spectra of the brightest Pleiades stars as well as CO maps toward the cluster show that there is gas as well as dust present and that the (primary) interstellar cloud has a significant radial velocity offset relative to the Pleiades (White 2003; Federman & Willson 1984). The gas and dust, therefore, are not a remnant from the formation of the cluster but are simply evidence of a transitory event as this small cloud passes by the cluster in our line of sight (see also Breger 1986). There are at least two claimed morphological signatures of a direct interaction of the Pleiades with the cloud. White & Bally (1993) provided evidence that the IRAS 60 and 100 m image of the vicinity of the Pleiades showed a dark channel immediately to the east of the Pleiades, which they interpreted as the "wake" of the Pleiades as it plowed through the cloud from the east. Herbig & Simon (2001) provided a detailed analysis of the optically brightest nebular feature in the Pleiades—IC 349 (Barnard's Merope nebula)—and concluded that the shape and structure of that nebula could best be understood if the cloud was running into the Pleiades from the southeast. Herbig & Simon (2001) concluded that the IC 349 cloudlet, and by extension the rest of the gas and dust enveloping the Pleiades, are relatively distant outliers of the Taurus molecular clouds (see also Eggen 1950 for a much earlier discussion ascribing the Merope nebulae as outliers of the Taurus clouds). White (2003) has more recently proposed a hybrid model, where there are two separate interstellar cloud complexes with very different space motions, both of which are colliding simultaneously with the Pleiades and with each other. Breger (1986) provided polarization measurements for a sample of member and background stars toward the Pleiades and argued that the variation in polarization signatures across the face of the cluster was evidence that some of the gas and dust was within the cluster. In particular, Figure 6 of that paper showed a fairly distinct interface region, with little residual polarization to the NE portion of the cluster and an L-shaped boundary running EW along the southern edge of the cluster and then north-south along the western edge of the cluster. Stars to the south and west of that boundary show relatively large polarizations and consistent angles (see also our Fig. 5, where we provide a few polarization vectors from Breger 1986 to illustrate the location of the interface region and the fact that the position angle of the polarization correlates well with the location in the interface). There is a general correspondence between the polarization map and what is seen with IRAC, in the sense that the B stars in the NE portion of the cluster (Atlas and Alcyone) have little nebular emission in their vicinity, whereas those in the western part of the cluster (Maia, Electra, and Asterope) have prominent, filamentary dust emission in their vicinity. The L-shaped boundary is in fact visible in Figure 4 as enhanced nebular emission running between and below a line roughly joining Merope and Electra and then making a right angle and running roughly parallel to a line running from Electra to Maia to HII 1234 (see Fig. 5). 6.1. Pleiades Dust-Star Encounters Imaged with IRAC The Pleiades dust filaments are most strongly evident in IRAC's 8 m channel, as evidenced by the distinct red color of the nebular features in Figure 4. The dominance at 8 m is an expected feature of reflection nebulae, as exemplified by NGC 7023 (Werner et al. 2004), where most of the mid-infrared emission arises from polycyclic aromatic hydrocarbons (PAHs) whose strongest bands in the 3–10 m region fall at 7.7 and 8.6 m. One might expect that if portions of the passing cloud were particularly near to one of the Pleiades members, it might be possible to identify such interactions by searching for stars with 8.0 m excesses or for stars with extended emission at 8 m. Figure 18 provides two such plots. Four stars stand out as having significant extended 8 m emission, with two of those stars also having an 8 m excess based on their [3.6] - [8.0] color. All of these stars, plus IC 349, are located approximately along the interface region identified by Breger (1986). Fig. 18 Two plots intended to (67 isolate Pleiades members with kB) excess and/or extended 8 m emission. The plot with [3.6] [8.0] m colors shows data from Table 3 (and hence is for aperture sizes of 3 pixel and 2 pixel radius, respectively). The increased vertical spread in the plots at faint magnitudes is simply due to decreasing signal-to-noise at 8 m. The numbers labeling stars with excesses are the HII identification numbers for those stars. We have subtracted a PSF from the 8 m images for the stars with extended emission, and those PSF-subtracted images are provided in Figure 19. The image for HII 1234 has the appearance of a bow shock. The shape is reminiscent of predictions for what one should expect from a collision between a large cloud or a sheet of gas and an A star as described in Artymowicz & Clampin (1997). The Artymowicz & Clampin model posits that A stars encountering a cloud will carve a paraboloidal shaped cavity in the cloud via radiation pressure. The exact size and shape of the cavity depend on the relative velocity of the encounter, the star's mass and luminosity and properties of the ISM grains. For typical parameters, the predicted characteristic size of the cavity is of order 1000 AU, quite comparable to the size of the structures around HII 652 and HII 1234. The observed appearance of the cavity depends on the view angle to the observer. However, in any case, the direction from which the gas is moving relative to the star can be inferred from the location of the star relative to the curved rim of the cavity; the "wind" originates approximately from the direction connecting the star and the apex of the rim. For HII 1234, this indicates the cloud that it is encountering has a motion relative to HII 1234 from the SSE, in accord with a Taurus origin and not in accord for where a cloud is impacting the Pleiades from the west as posited in White (2003). The nebular emission for HII 652 is less strongly bow-shaped, but the peak of the excess emission is displaced roughly southward from the star, consistent with the Taurus model and inconsistent with gas flowing from the west. Fig. 19 Postage stamp images extracted from individual, 8 m BCDs for the stars with extended 8 m emission, from which we have subtracted an empirical PSF. Clockwise from the upper left, the stars shown are HII 1234, HII 859, Merope, and HII 652. The fivepointed star indicates the astrometric position of the star (often superposed on a few black pixels where the 8 m image was saturated. The circle in the Merope image is centered on the location of IC 349 and has a diameter of about 25 (the size of IC 349 in the optical is of order 10 × 10 ). Despite being the brightest part of the Pleiades nebulae in the optical, IC 349 appears to be undetected in the 8 m image. This is not because the 8 m image is insensitive to the nebular emission—there is generally good agreement between the structures seen in the optical and at 8 m, and most of the filaments present in optical images of the Pleiades are also visible on the 8 m image (see Figs. 4 and 19) and even the PSF-subtracted image of Merope shows well-defined nebular filaments. The lack of enhanced 8 m emission from the region of IC 349 is probably because all of the small particles have been scoured away from this cloudlet, consistent with Herbig's model to explain the HST surface photometry and colors. There is no PAH emission from IC 349 because there are none of the small molecules that are the postulated source of the PAH emission. IC 349 is very bright in the optical, and undetected to a good sensitivity limit at 8 m; it must be detectable via imaging at some wavelength between 5000 Å and 8 m. We checked our 3.6 m data for this purpose. In the standard BCD mosaic image, we were unable to discern an excess at the location of IC 349 either simply by displaying the image with various stretches or by doing cuts through the image. We performed a PSF subtraction of Merope from the image in order to attempt to improve our ability to detect faint, extended emission 30 from Merope—unfortunately, bright stars have ghost images in IRAC channel 1, and in this case the ghost image falls almost exactly at the location of IC 349. IC 349 is also not detected in visual inspection of our 2MASS 6x images. 6.2. Circumstellar Disks and IRAC As part of the Spitzer FEPS (Formation and Evolution of Planetary Systems) Legacy program, using pointed MIPS photometry, Stauffer et al. (2005) identified three G dwarfs in the Pleiades as having 24 m excesses probably indicative of circumstellar dust disks. Gorlova et al. (2006) reported results of a MIPS GTO survey of the Pleiades and identified nine cluster members that appear to have 24 m excesses due to circumstellar disks. However, it is possible that in a few cases these apparent excesses could be due instead to a knot of the passing interstellar dust impacting the cluster member or that the 24 m excess could be flux from a background galaxy projected onto the line of sight to the Pleiades member. Careful analysis of the IRAC images of these cluster members may help confirm that the MIPS excesses are evidence for debris disks rather than the other possible explanations. Six of the Pleiades members with probable 24 m excesses are included in the region mapped with IRAC. However, only four of them have data at 8 m—the other two fall near the edge of the mapped region and only have data at 3.6 and 5.8 m. None of the six stars appear to have significant local nebular dust from visual inspection of the IRAC mosaic images. Also, none of them appear problematic in Figure 18. For a slightly more quantitative analysis of possible nebular contamination, we also constructed aperture growth curves for the six stars and compared them to other Pleiades members. All but one of the six show aperture growth curves that are normal and consistent with the expected IRAC PSF. The one exception is HII 489, which has a slight excess at large aperture sizes, as is illustrated in Figure 20. Because HII 489 only has a small 24 m excess, it is possible that the 24 m excess is due to a local knot of the interstellar cloud material and is not due to a debris disk. For the other five 24 m excess stars we find no such problem, and we conclude that their 24 m excesses are indeed best explained as due to debris disks. Fig. 20 Aperture growth curves from the 8 m mosaic for stars with 24 m excesses from Gorlova et al. (2006) and for a set of control objects (dashed curves). All of the objects have been scaled to common zero-point magnitudes for 9 pixel apertures, with the 24 m excess stars offset from the control objects by 0.1 mag. The three Gorlova et al. (2006) stars with no excess at 8 m are HII 996, HII 1284, and HII 2195. The Gorlova et al. (2006) star with a slight excess at 8 m is HII 489. (52 kB) 7. SUMMARY AND CONCLUSIONS We have collated the primary membership catalogs for the Pleiades to produce the first catalog of the cluster extending from its highest mass members to the substellar limit. At the bright end, we expect this catalog to be essentially complete and with few or no nonmember contaminants. At the faint end, the data establishing membership are much sparser, and we expect a significant number of objects will be nonmembers. We hope that the creation of this catalog will spur efforts to obtain accurate radial velocities and proper motions for the faint candidate members in order to eventually provide a well-vetted membership catalog for the stellar members of the Pleiades. Toward that end, it would be useful to update the current catalog with other data—such as radial velocities, lithium equivalent widths, X-ray fluxes, H equivalent widths, etc.—which could be used to help accurately establish membership for the low-mass cluster candidates. It is also possible to make more use of "negative information" present in the proper-motion catalogs. That is, if a member from one catalog is not included in another study but does fall within its areal and luminosity coverage, that suggests that it likely failed the membership criteria of the second study. For a few individual stars, we have done this type of comparison, but a systematic analysis of the proper-motion catalogs should be conducted. We intend to undertake these tasks and plan to establish a Web site where these data would be hosted. We have used the new Pleiades member catalog to define the single-star locus at 100 Myr for BVICKs and the four IRAC bands. These curves can be used as empirical calibration curves when attempting to identify members of less well-studied, more distant clusters of similar age. We compared the Pleiades photometry to theoretical isochrones from Siess et al. (2000) and Baraffe et al. (1998). The Siess et al. (2000) isochrones are not, in detail, a good fit to the Pleiades photometry, particularly for low-mass stars. The Baraffe et al. (1998) 100 Myr isochrone does fit the Pleiades photometry very well in the I versus I - K plane. We have identified 31 new substellar candidate members of the Pleiades using our combined seven-band infrared photometry and have shown that the majority of these objects appear to share the Pleiades proper motion. We believe that most of the objects that may be contaminating our list of candidate brown dwarfs are likely to be unresolved galaxies, and therefore low-resolution spectroscopy should be able to provide a good criterion for culling our list of nonmembers. The IRAC images, particularly the 8 m mosaic, provide vivid evidence of the strong interaction of the Pleiades stars and the interstellar cloud that is passing through the Pleiades. Our data are supportive of the model proposed by Herbig & Simon (2001) whereby the passing cloud is part of the Taurus cloud complex and hence is encountering the Pleiades from the SSE direction. White & Bally (1993) had proposed a model whereby the cloud was encountering the Pleiades from the west and used this to explain features in the IRAS 60 and 100 m images of the region as the wake of the Pleiades moving through the cloud. Our data appear to not be supportive of that hypothesis and therefore leave the apparent structure in the IRAS maps as unexplained. Most of the support for this work was provided by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. This research has made use of NASA's Astrophysics Data System (ADS) Abstract Service, and of the SIMBAD database, operated at CDS, Strasbourg, France. This research has made use of data products from the Two Micron All-Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation. These data were served by the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The research described in this paper was partially carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research made use of the SIMBAD database operated at CDS, Strasbourg, France, and also of the NED and NStED databases operated at IPAC, Pasadena, CA. A large amount of data for the Pleiades (and other open clusters) can also be found at the open cluster database WEBDA (http://www.univie.ac.at/webda/), operated in Vienna by Ernst Paunzen. APPENDIX A1. MEMBERSHIP CATALOGS Membership lists of the Pleiades date back to antiquity if one includes historical and literary references to the Seven Sisters (Alcyone, Maia, Merope, Electra, Taygeta, Asterope, and Celeno) and their parents (Atlas and Pleione). The first paper discussing relative proper motions of a large sample of stars in the Pleiades (based on visual observations) was published by Pritchard (1884). The best of the early proper-motion surveys of the Pleiades derived from photographic plate astrometry was that by Trumpler (1921), based on plates obtained at Yerkes and Lick observatories. The candidate members from that survey were presented in two tables, with the first being devoted to candidate members within about 1° from the cluster center (operationally, within 1° from Alcyone) and the second table being devoted to candidates further than 1° from the cluster center. Most of the latter stars were denoted by Trumpler by an S or R, followed by an identification number. We use Tr to designate the Trumpler stars (hence Trnnn for a star from the first table and the small number of stars in the second table without an "S" or an "R," and TrSnnn or TrRnnn for the other stars). For the central region, Trumpler's catalog extends to V 13, while the outer region catalog includes stars only to about V 9. The most heavily referenced proper-motion catalog of the Pleiades is that provided by Hertzsprung (1947). That paper makes reference to two separate catalogs: a photometric catalog of the Pleiades published by Hertzsprung (1923), whose members are commonly referred to by HI numbers, and the new proper-motion catalog from the 1947 paper, commonly referenced as the HII catalog. While both HI and HII numbers have been used in subsequent observational papers, it is the HII identification numbers that predominate. That catalog—derived from Carte du Ciel blue-sensitive plates from 14 observatories—includes stars in the central 2 × 2 region of the cluster and has a faint limit of about V = 15.5. Johnson system BVI photometry is provided for most of the proposed Hertzsprung members in Johnson & Mitchell (1958) and Iriarte (1967). Additional Johnson B and V photometry plus Kron I photometry for a fairly large number of the Hertzsprung members can be found in Stauffer (1980, 1982, 1984). Other Johnson BV photometry for a scattering of stars can be found in Jones (1973), Robinson & Kraft (1974), and Messina (2001). Spectroscopic confirmation, primarily via radial velocities, that these are indeed Pleiades members has been provided in Soderblom et al. (1993), Queloz et al. (1998), and Mermilliod et al. (1997). Two other proper-motion surveys provide relatively bright candidate members relatively far from the cluster center: Artyukhina & Kalinina (1970) and van Leeuwen 1986. Stars from the Artyukhina catalog are designated as "AK" followed by the region from which the star was identified followed by an identification number. The new members provided in the van Leeuwen paper were taken from an otherwise unpublished proper-motion study by Pels, where the first 118 stars were considered probable members and the remaining 75 stars were considered possible members. Van Leeuwen categorized a number of the Pels stars as nonmembers based on the Walraven photometry they obtained, and we adopt those findings. Radial velocities for stars in these two catalogs have been obtained by Rosvick et al. (1992), Mermilliod et al. (1997), and Queloz et al. (1998), and those authors identified a list of the candidate members that they considered confirmed by the high-resolution spectroscopy. For these outlying candidate members, to be included in Table 2 we require that the star be a radial velocity member from one of the above three surveys, or be indicated as having "no dip" in the Coravel cross-correlation (indicating rapid rotation, which at least for the later type stars is suggestive of membership). Geneva photometry of the Artyukhina stars considered as likely members was provided by Mermilliod et al. (1997). The magnitude limit of these surveys was not well-defined, but most of the Artyukhina and Pels stars are brighter than V = 13. Jones (1973) provided proper-motion membership probabilities for a large sample of proposed Pleiades members, and for a set of faint, red stars toward the Pleiades. A few star identification names from the sources considered by Jones appear in Table 2, including MT (McCarthy & Treanor 1964), VM (van Maanen 1946), and ALR (Ahmed et al. 1965; Jones 1973). The chronologically next significant source of new Pleiades candidate members was the flare star survey of the Pleiades conducted at several observatories in the 1960s, and summarized in Haro et al. (1982, hereafter HCG). The logic behind these surveys was that even at 100 Myr, late-type dwarfs have relatively frequent and relatively high-luminosity flares (as demonstrated by Johnson & Mitchell 1958 having detected two flares during their photometric observations of the Pleiades), and therefore wide area, rapid cadence imaging of the Pleiades at blue wavelengths should be capable of identifying low-mass cluster members. However, such surveys also will detect relatively young field dwarfs, and therefore it is best to combine the flare star surveys with proper motions. Dedicated proper-motion surveys of the HCG flare stars were conducted by Jones (1981) and Stauffer et al. (1991), with the latter also providing photographic VI photometry (Kron system). Photoelectric photometry for some of the HCG stars have been reported in Stauffer (1982, 1984), Stauffer & Hartmann (1987), and Prosser et al. (1991). High-resolution spectroscopy of many of the HCG stars is reported in Stauffer (1984), Stauffer & Hartmann (1987), and Terndrup et al. (2000). Because a number of the papers providing additional observational data for the flare stars were obtained prior to 1982, we also include in Table 2 the original flare star names that were derived from the observatory where the initial flare was detected. Those names are of the form of an initial letter indicating the observatory—A (Asiago), B (Byurakan), K (Konkoly), T (Tonantzintla)—followed by an identification number. Stauffer et al. (1991) conducted two proper-motion surveys of the Pleiades over an approximately 4 × 4 region of the cluster based on plates obtained with the Lick 20 astrographic telescope. The first survey was essentially unbiased, except for the requirement that the stars fall approximately in the region of the V versus V - I color-magnitude diagram where Pleiades members should lie. Candidate members from this survey are designated by SK numbers. The second survey was a proper-motion survey of the HCG stars. Photographic VI photometry of all the stars was provided as well as proper-motion membership probabilities. Photoelectric photometry for some of the candidate members was obtained as detailed above in the section on the HCG catalog stars. The faint limit of these surveys is about V = 18. Hambly et al. (1991) provided a significantly deeper, somewhat wider area proper-motion survey, with the faintest members having V 20 and the total area covered being of order 25 deg2. The survey utilized red sensitive plates from the Palomar and UK Schmidt telescopes. Due to incomplete coverage at one epoch, there is a vertical swath slightly east of the cluster center where no membership information is available. Stars from this survey are designated by their HHJ numbers. Hambly et al. (1993) provide RI photographic photometry on a natural system for all of their candidate members, plus photoelectric Cousins RI photometry for a small number of stars and JHK photometry for a larger sample. Some spectroscopy to confirm membership has been reported in Stauffer et al. (1994, 1995, 1999), Oppenheimer et al. (1997), and Steele et al. (1995), although for most of the HHJ stars there is no spectroscopic membership confirmation. Pinfield et al. (2000) provide the deepest wide-field proper-motion survey of the Pleiades. That survey combines CCD imaging of 6 deg2 of the Pleiades obtained with the Burrell Schmidt telescope (as five separate, nonoverlapping fields near but outside the cluster center) with deep photographic plates that provide the first epoch positions. Candidate members are designated by BPL numbers (for Burrell Pleiades), with the faintest stars having I 19.5, corresponding to V > 23. Only the stars brighter than about I = 17 have sufficiently accurate proper motions to use to identify Pleiades members. Fainter than I = 17, the primary selection criteria are that the star fall in an appropriate place in both an I versus I - Z and an I versus I - K CMD. Adams et al. (2001) combined the 2MASS and digitized POSS databases to produce a very wide area propermotion survey of the Pleiades. By design, that survey was very inclusive—covering the entire physical area of the cluster and extending to the hydrogen-burning mass limit. However, it was also very "contaminated," with many suspected nonmembers. The catalog of possible members was not published. We have therefore not included stars from this study in Table 2; we have used the proper-motion data from Adams et al. (2001) to help decide cases where a given star has ambiguous membership data from the other surveys. Deacon & Hambly (2004) provided another deep and very wide area proper-motion survey of the Pleiades. The survey covers a circular area of approximately 5° radius to R 20, or V 22. Candidate members are designated by "DH." Deacon & Hambly (2004) also provide membership probabilities based on proper motions for many candidate cluster members from previous surveys. For stars where Deacon & Hambly (2004) derive P < 0.1 and where we have no other proper-motion information or where another proper-motion survey also finds low membership probability, we exclude the star from our catalog. For cases where two of our proper-motion catalogs differ significantly in their membership assessment, with one survey indicating the star is a probable member, we retain the star in the catalog as the conservative choice. Examples of the latter where Deacon & Hambly (2004) derive P < 0.1 include HII 1553, HII 2147, HII 2278, and HII 2665—all of which we retain in our catalog because other surveys indicate these are high-probability Pleiades members. A2. PHOTOMETRY Photometry for stars in open cluster catalogs can be used to help confirm cluster membership and to help constrain physical properties of those stars or of the cluster. For a variety of reasons, photometry of stars in the Pleiades has been obtained in a panoply of different photometric systems. For our own goals, which are to use the photometry to help verify membership and to define the Pleiades single-star locus in color-magnitude diagrams, we have attempted to convert photometry in several of these systems to a common system (Johnson BV and Cousins I). We detail below the sources of the photometry and the conversions we have employed. Photoelectric photometry of Pleiades members dates back to at least 1921 Cummings (1921). However, as far as we are aware the first "modern" photoelectric photometry for the Pleiades, using a potassium hydride photoelectric cell, is that of Calder & Shapley (1937). Eggen (1950) provided photoelectric photometry using a 1P21 phototube (but calibrated to a no-longer-used photographic system) for most of the known Pleiades members within 1° of the cluster center and with magnitudes <11. The first phototube photometry of Pleiades stars calibrated more-or-less to the modern UBV system was provided by Johnson & Morgan (1951). An update of that paper, and the oldest photometry included here was reported in Johnson & Mitchell (1958), which provided UBV Johnson system photometry for a large sample of HII and Trumpler candidate Pleiades members. Iriarte (1967) later reported Johnson system V - I colors for most of these stars. We have converted Iriarte's V - I photometry to estimated Cousins V - I colors using a formula from Bessell (1979): BVRI photometry for most of the Hertzsprung members fainter than V = 10 has been published by Stauffer (1980, 1982, 1984) and Stauffer & Hartmann (1987). The BV photometry is Johnson system, whereas the RI photometry is on the Kron system. The Kron V - I colors were converted to Cousins V - I using a transformation provided by Bessell & Weis (1987): Other Kron system V - I colors have been published for Pleiades candidates in Stauffer et al. (1991, photographic photometry) and in Prosser et al. (1991). These Kron-system colors have also been converted to Cousins V - I using the above formula. Johnson/Cousins UBVR photometry for a set of low-mass Pleiades members was provided by Landolt (1979). We only use the BV magnitudes from that study. Additional Johnson system UBV photometry for small numbers of stars is provided in Robinson & Kraft (1974), Messina (2001), and Jones (1973). Van Leeuwen et al. (1987) provided Walraven VBLUW photometry for nearly all of the Hertzsprung members brighter than V 13.5 and for the Pels candidate members. Van Leeuwen provided an estimated Johnson V derived from the Walraven V in his tables. We have transformed the Walraven V - B color into an estimate of Johnson B - V using a formula from Rosvick et al. (1992): Hambly et al. (1993) provided photographic VRI photometry for all of the HHJ candidate members and VRI Cousins photoelectric photometry for a small fraction of those stars. We took all of the HHJ stars with photographic photometry for which we also have photoelectric VI photometry on the Cousins system, and plotted V(Cousins) versus V(HHJ) and I(Cousins) versus I(HHJ). While there is some evidence for slight systematic departures of the HHJ photographic photometry from the Cousins system, those departures are relatively small and we have chosen simply to retain the HHJ values and treat them as Cousins system. Pinfield et al. (2000) reported their I magnitudes in an instrumental system that they designated as Ikp. We identified all BPL candidate members for which we had photoelectric Cousins I estimates, and plotted Ikp versus IC. Figure 21 shows this correlation, and the piecewise linear fit we have made to convert from Ikp to IC. Our catalog lists these converted IC measures for the BPL stars for which we have no other photoelectric I estimates. Fig. 21 Calibration derived relating Ikp from Pinfield et al. (2000) and IC. The dots represent stars for which we have both Ikp and IC measurements (small dots: photographic IC; large dots: photoelectric IC), and the solid line indicates the piecewise linear fit we use to convert the Ikp values to IC for stars for which we only have Ikp. Deacon & Hambly (2004) derived RI photometry from the scans of their plates and calibrated that photometry by reference to published photometry from the literature. When we plotted their the difference between their I-band photometry and literature values (where available), we discovered a significant dependence on right ascension. Unfortunately, because the DH survey extended over larger spatial scales than the calibrating photometry, we could not derive a correction that we could apply to all the DH stars. We therefore developed the following indirect scheme. We used the stars for which we have estimated IC magnitudes (from photoelectric photometry) to define the relation between J and (IC - J) for Pleiades members. For each DH star, we combined that relation and the 2MASS J magnitude to yield a predicted IC. Figure 22 shows a plot of the difference of this predicted IC and I(DH) with right ascension. The solid line shows the relation we adopt. Figure 23 shows the relation between the corrected I(DH) values and Table 2 IC measures from photoelectric sources. There is still a significant amount of scatter, but the corrected I(DH) photometry appears to be accurately calibrated to the Cousins system. Fig. 22 Difference between the predicted IC and Deacon & Hambly (2004) I magnitude as a function of right ascension for the DH stars. No obvious dependence is present vs. declination. Fig. 23 Comparison of the recalibrated DH I photometry with estimates of IC for stars in Table 2 with photoelectric data In a very few cases (specifically, just five stars), we provide an estimate of IC based on data from a wide-area CCD survey of Taurus obtained with the Quest-2 camera on the Palomar 48 inch Samuel Oschin telescope (Slesnick et al. 2006). That survey calibrated their photometry to the Sloan i system, and we have converted the Sloan i magnitudes to IC. We intend to make more complete use of the Quest-2 data in a subsequent paper. When we have multiple sources of photometry for a given star, we consider how to combine them. In most cases, if we have photoelectric data, that is given preference. However, if we have photographic V and I, and only a photoelectric measurement for I, we do not replace the photographic I with the photoelectric value because these stars are variable and the photographic measurements are at least in some cases from nearly simultaneous exposures. Where we have multiple sources for photoelectric photometry, and no strong reason to favor one measurement or set of measurements over another, we have averaged the photometry for a given star. In most cases where we have multiple photometry the individual measurements agree reasonably well but with the caveat that the Pleiades low-mass stars are in many cases heavily spotted and "active" chromospherically and hence are photometrically variable. In a few cases, even given the expectation that spots and other phenomena may affect the photometry, there seems to be more discrepancy between reported V magnitudes than we expect. We note two such cases here. 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Después del trabajo de campo en tierras chiapanecas, Guillermo Bernal Romero, del Centro de Estudios Mayas del Instituto Filológicas (IIFL) de la UNAM, (México), volvió a su cubículo y descifró el mensaje; la existencia de este Ciclo que había pasado desapercibido en los estudios clásicos en torno al calendario. Al hacer la reconstrucción, Bernal comprobó que el período estuvo asociado con el ritual de Taladrado de fuego, (joch´K´ahk´), es decir, de generación por fricción de un fuego ritual dedicado al Dios Zarigüeya o Tlacuache. El Ciclo 63 es una especie de eslabón perdido de un engranaje que faltaba. Se conocían otros de 7; 9, y 819 días. El descubierto en Abril pasado es el resultado de multiplicar los primeros 2, y el tercero, de multiplicar 819 por 13. Estos números no fueron un capricho de los Mayas, eran sagrados: Creían en la existencia de un Supra mundo o región celeste con 13 niveles; de una terrestre, (la nuestra), con 7 estratos; y un inframundo con 9 niveles. Respecto al 819, se ha propuesto que fue formulado para realizar cómputos de los Períodos Sinódicos, (Tiempo que tarda un objeto en volver a aparecer en el mismo punto del cielo, con respecto al Sol, al observarlo desde la Tierra), de Saturno, de 378 días, (63 X 6). En 1993, Arnaldo González Cruz, Director del Proyecto Arqueológico Palenque, del Instituto Nacional de Antropología e Historia, de la UNAM, descubrió entre los restos del Edificio I del grupo XVI, conjunto habitacional sacerdotal, ubicado a un lado del Corazón Ceremonial de la ciudad, los fragmentos de lo que parecía un tablero. Se encontraban dispersos, sepultados entre los escombros de la derruida construcción, donde los pedazos del estuco, en el Período Clásico, en la época de K´inich Janahb´Pakal Il, “El Grande”, cubrieron las paredes de 2 pilastras. Solo algunos cartuchos glíficos estaban pegados a las pilastras en su posición original. Bernal Romero hizo un primer estudio de estos fragmentos en 1998. Allí descubrió un registro de 819 días. En 2013 hubo una segunda revisión del material, pero no fue hasta abril de 2014 que la restauradora Luz Lourdes Herbert, de la Coordinación Nacional de Conservación del Patrimonio Cultural del INAH, desplegó completamente el material en camas de arena. Foto: A pesar del desarrollo de la Epigrafía Maya, y del desciframiento de los acontecimientos históricos o míticos que relatan las inscripciones, el calendario aún tiene aspectos insospechados. Foto: UNAM) Ya extendidos los cuadros de escritura, se determinó que se trataba de 2 tableros que estuvieron colocados sobre jambas, pero las piezas estaban revueltas y no se sabía que cartuchos pertenecían a uno u otro “rompecabezas”. Eso causó Problemas, pero al observar con más detenimiento se pudo realizar la separación fina. Coincidían y tenían sentido. Por ejemplo, con el dato del glifo del Dios Zarigüeya, en el extremo superior derecho del Tablero Este, se podía saber cuántos cartuchos había tenido todo; cuatro columnas (dos dobles) y 14 filas, es decir, 56 espacios de escritura. Además, el nombre de la deidad va acompañado de otros glifos, como el fuego, y antes de un verbo. A partir de una esquina se reconstruyó todo, y aunque quedaron algunos huecos, donde ya no existen glifos, se pudo determinar que hubo allí El tablero Oeste se recuperó en un 30% y el Este en un 65%. La reconstrucción fue posible por la lógica del texto del cómputo que posee las formulas bien conocidas de los ciclos calendáricos Mayas. El segundo comprende una fecha absoluta de cuenta larga, que en nuestro calendario equivales al 28 de junio de 673; de esta los Mayas hicieron un cómputo hacia la fecha anterior, el 28 de mayo, 31 días antes, (habían transcurrido 11 días y un winal…), cuando se taladró el fuego, dedicado a la deidad Zarigüeya o el Tlacuache. Esa ceremonia es muy significativa en el pensamiento mesoamericano; en la mitología, a ese animal se le atribuye haber robado el fuego para dárselo al hombre. Se conocía que los Mayas hacían esta ceremonia de manera sacrilizada, pero ahora se sabe que se realizaban periódicamente cada 63 días. La comprobación del hecho se hizo en otro monumento, el dintel 29 de Yaxtilán, donde se observó que un rito de taladro para el mismo Dios se ocurría en un lapso múltiplo de 63 con respecto al registro en Palenque, es decir 13.230 días. (210 X 63). Debido a que podía tratarse de una casualidad, se buscó otros registros, encontrándose al menos 8, como el Panel 2 de Laxtinich. El intervalo entre este y la fecha de Yaxchilán es equivalente a 345 ciclos de 63 días, es decir 21.735 días. Esta periodicidad no podía ser casual, sino intencional. Además, es posible que este ciclo se haya utilizado para estimar el período Sinódico de Saturno, que es de 378 días. El ciclo 63 no fue registrado con frecuencia por los Mayas, lo que explica por qué paso desapercibido. No había mucho elementos pero la reconstrucción de los tableros, en especial el Este, dio la pista para llegar a este período, que explica cómo se construyeron otros factores numéricos tipo calendáricos. Eric Thompson en 1943 descubrió que el 819 era resultado de la multiplicación de 3 cifras, 7; 9; y 13, hoy se sabe que no es de manera serial, sino segmentada, es decir, 9 X 7; y luego 63 X 13. Para los investigadores es fascinante, pues ahora saben que existen relaciones numéricas insospechadas que delatan la existencia de otros ciclos. En otras palabras, la compleja maquinaria numérica que se creía resuelta aún no lo está. Revelan por qué la Luna no es una esfera perfecta La Luna se sitúa a una distancia media de la Tierra de 384.000 km y se aleja de ella unos 3,8 centímetros por año. Investigadores afirman que el satélite sería ligeramente achatado producto de las primeras fuerzas de marea ejercidas por la Tierra hace 4,4 millardos de años Llena, en cuarto creciente o menguante, la Luna, por conocida que resulte para los terrícolas, tiene sus misterios. Un equipo de investigadores propone en la revista Nature una explicación a su forma, que no es la de una esfera perfecta. El satélite natural de la Tierra no es exactamente esférico, sino ligeramente achatado. La Luna presenta una ligera hinchazón en su cara visible desde la Tierra, y otra en la cara oculta. El equipo de Ian Garrick-Bethell, de la Universidad de California, explica la forma particular por los “efectos de marea”, las fuerzas gravitacionales ejercidas por la Tierra durante la infancia de la Luna, hace 4,4 millardos de años. El Sistema Solar se formó hace aproximadamente 4,5 millardos de años. Conforme al modelo que hoy es corrientemente admitido, la Luna habría nacido de una colisión masiva padecida por la Tierra, que se acababa de formar. Según los investigadores, las primeras fuerzas de marea ejercidas por la Tierra, que entonces estaba mucho más cercana a la Luna, calentaron de forma desigual, según los lugares, la corteza de la Luna, cuando entonces ésta era un océano de rocas en fusión. Este fenómeno dio a la Luna su forma, ligeramente alargada como un limón. Más tarde, cuando la Luna se enfriaba, las fuerzas de las mareas deformaron el exterior de la Luna y fijaron sus irregularidades. La luna se sitúa a una distancia media de la Tierra de 384.000 km y se aleja de ella unos 3,8 centímetros por año. Su circunferencia en el ecuador es de 10.920 km, es decir 3,7 veces inferior a la de la Tierra (40.000 km). Scientists discover vast methane plumes escaping from Arctic seafloor Scientists aboard the icebreaker Oden observe a methane mega flare. Methane mega flare event on the Laptev Sea slope of the Arctic Ocean, at a depth of about 62 meters. Image via Daily Kos via University of Stockholm. An international team of scientists aboard the icebreaker Oden – currently north of eastern Siberia, in the Arctic Ocean – is working primarily to measure methane emissions from the Arctic seafloor. On July 22, 2014, only a week into their voyage, the team reported “elevated methane levels, about 10 times higher than background seawater.” They say the culprit in this release of methane, a potent greenhouse gas, may be a tongue of relatively warm water from the Atlantic Ocean, the last remnants of the Gulf Stream, mixing into the Arctic Ocean. A press release from University of Stockholm described the discovery as: … vast methane plumes escaping from the seafloor of the Laptev continental slope. These early glimpses of what may be in store for a warming Arctic Ocean could help scientists project the future releases of the strong greenhouse gas methane from the Arctic Ocean. The scientists refer to the plumes as methane mega flares. Expedition of the icebreaker Oden – called the SWERUS expedition – preliminary cruise plan and study areas of Leg 1 and 2. EEZ=Exclusive Economic Zone; LR=Lomonosov Ridge; MR=Mendeleev Ridge; HC=Herald Canyon; NSI=New Siberian Islands. Image via Daily Kos via University of Stockholm. On July 22, 2014, chief scientist Örjan Gustafsson of the University of Stockholm wrote about the methane mega flare event in his blog. He wrote: So, what have we found in the first couple of days of methane-focused studies? 1) Our first observations of elevated methane levels, about ten times higher than in background seawater, were documented already as we climbed up the steep continental slope at stations in 500 and 250 meter depth. This was somewhat of a surprise. While there has been much speculation of the vulnerability of regular marine hydrates [frozen methane formed due to high pressure and low temperature] along the Arctic rim, very few actual observations of methane releases due to collapsing Arctic upper slope marine hydrates have been made. ¨ It has recently been documented that a tongue of relatively warm Atlantic water, with a core at depths of 200–600 meters may have warmed up some in recent years. As this Atlantic water, the last remnants of the Gulf Stream, propagates eastward along the upper slope of the East Siberian margin, our SWERUS-C3 program is hypothesizing that this heating may lead to destabilization of upper portion of the slope methane hydrates. This may be what we now for the first time are observing. 2) Using the mid-water sonar, we mapped out an area of several kilometers where bubbles were filling the water column from depths of 200 to 500 meters. During the preceding 48 hours we have performed station work in two areas on the shallow shelf with depths of 60-70m where we discovered over 100 new methane seep sites. SWERUS-C3 researchers have on earlier expeditions documented extensive venting of methane from the subsea system to the atmosphere over the East Siberian Arctic Shelf. On this Oden expedition we have gathered a strong team to assess these methane releases in greater detail than ever before to substantially improve our collective understanding of the methane sources and the functioning of the system. This is information that is crucial if we are to be able to provide scientific estimations of how these methane releases may develop in the future. While not as long-lasting in the atmosphere as carbon dioxide, methane is much more effective than carbon dioxide at trapping heat. Glaciologist Jason Box, in a recent and fascinating blog post (Is the climate dragon awakening?) said: “Atmospheric methane release is a much bigger problem than atmospheric carbon dioxide release, since methane is ~20 times more powerful greenhouse gas”. Methane has the potential to create a feedback loop in global warming. That is, as Earth’s climate warms, methane that is frozen in reservoirs stored in Arctic tundra soils – or marine sediments – may be released into the atmosphere. It does not last long in the atmosphere (on the order of years, rather than centuries as with carbon dixoide). But its release will cause more warming, which will cause more methane to be released, replenishing that in the atmosphere … causing more warming and more methane release and so on. Methane release from the Arctic Ocean is not a new phenomenon; after all, the Stockholm scientists were there to measure it. U.S. scientists have observed Arctic Ocean methane release, too. For example, NASA reported in April 2012 on a study in which scientists measured surprising levels of methane coming from cracks in Arctic sea ice and areas of partial sea ice cover. In 2013, Shakova et al (2013) suggested that: … significant quantities of methane are escaping the East Siberian Shelf as a result of the degradation of submarine permafrost over thousands of years. We suggest that bubbles and storms facilitate the flux of this methane to the overlying ocean and atmosphere, respectively. Glaciologist Jason Box, in his recent blog post, pointed out that methane release tends to come in spikes, which he calls “dragon’s breath.” Jason Box published the chart above in his blog. It shows a possible methane spike. Box said: “A reasonable hypothesis for the outliers [apparent high measurements of methane, which Box calls 'dragon's breath'] … would be: extreme outlying positive anomalies represent high methane concentration plumes emanating from tundra and/or oceanic sources. Another reasonable hypothesis would be: extreme outlying positive anomalies represent observational errors. What NOAA states: the outliers ‘are thought to be not indicative of background conditions, and represent poorly mixed air masses influenced by local or regional anthropogenic sources or strong local biospheric sources or sinks.’ Fair enough. But the dragon breath hypothesis has me losing sleep.” Methane bubbles discovered on Laptev continental slope of Arctic Ocean by the science team aboard the icebreaker Oden. Image via University of Stockholm. On July 23, Ulf Hedman – who is aboard the Oden and who is Science Coordinator for the Swedish Polar Research Secretariat – gave a vivid description of the discovery in his blog: We are ‘sniffing’ methane. We see the bubbles on video from the camera mounted on the CTD or the Multicorer. All analysis tells the signs. We are in a [methane] mega flare. We see it in the water column we read it above the surface an we follow it up high into the sky with radars and lasers. We see it mixed in the air and carried away with the winds. Methane in the air. Where does it come from? Is it from the old moors and mosses that used to be on dry land but now has sunken into the sea. Does it come from the deep interior of the Earth following structures in the bedrock up into the sand filled reservoirs collecting oil and gas then leaking out upwards, as bubbles through the sea bed into the water, into the mid-water sonar, the Niskin bottles the analysis and into our results? Where does the methane come from? Is it organic or not? What’s the volume? How much is carried up into the air? Is there an effect on the climate? One mega flare does not tell the truth. It’s not evidence enough. We carry on for the next station. And the next, and next, next… Bottom line: A team of international scientists aboard the icebreaker Oden has documented “elevated methane levels, about ten times higher than in background seawater” in the Arctic Ocean. They are calling it a methane mega flare event and express hopes it will help them project future releases of the strong greenhouse gas methane from the Arctic Ocean, and to understand the role this released methane might play in global warming. Follow the SWERUS-C3 expedition – @SWERUSC3 – on Twitter. Mystery crater in Yamal peninsula probably caused by methane release Científicos captan olas de cinco metros en Océano Ártico Jim Thomson, Universidad de Washington El fenómeno sería consecuencia del calentamiento global, y acentuaría el derretimiento de hielo dentro de la zona Un nuevo y preocupante fenómeno ha sido captado en el Polo Norte, específicamente, dentro del Océano Ártico, donde no sólo se han registrado importantes niveles de deshielo sino que ahora también, y por primera vez, olas de cinco metros de altura. El suceso fue captado por el experto de la Universidad de Washington, Jim Thomson, quien detectó durante septiembre de 2012 estas importantes olas generadas por el viento, que no sólo serían una evidencia del calentamiento global sino que podrían ser la causa, además, de un derretimiento más acelerado en el hielo de esta zona. La investigación de Thomson muestra que durante 2012, se llegaron a formar olas de hasta cinco metros de altura durante la parte más fuerte de una tormenta, que habrían surgido de vientos habituales en la zona pero con una nueva realidad de mar abierto mucho más amplio en la zona. Este fenómeno también responde al retroceso del hielo ártico durante el verano, que sucede en un promedio habitual de 150 kilómetros de la costa. Sin embargo, en 2012, esta cifra saltó a los 1.500 kilómetros, permitiendo una mayor temporada de mar abierto y por tanto, un escenario más proclive a generar olas más altas. Según los expertos, este fenómeno podría significar un importante cambio en las condiciones históricas de esta zona de hielo y podría traer consecuencias dentro de ese ecosistema como también dentro de la navegación. El científico junto a un grupo de otros expertos esperan evaluar más a detalle este fenómeno dentro de la zona, con una serie de instrumentos que serán ubicados en Alaska durante los meses de verano. Old pre-main-sequence stars Disc reformation by Bondi-Hoyle accretion 1,2 3,4,5 4 2,4,6 P. Scicluna , G. Rosotti , J. E. Dale and L. Testi 1 ITAP, Universität zu Kiel, Leibnizstr. 15, 24118 Kiel, Germany e-mail: [email protected] 2 European Southern Observatory, Karl-Schwarzschild-Str. 85748 Garching b München, Germany 3 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany 4 Excellence Cluster Universe, Boltzmannstr. 85748 Garching, Germany 5 Universitats-Sternwarte München, Scheinerstraße81679 München, Germany 6 INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy Abstract Young stars show evidence of accretion discs which evolve quickly and disperse with an e-folding time of ~3 Myr. This is in striking contrast with recent observations that suggest evidence of numerous >30 Myr old stars with an accretion disc in large star-forming complexes. We consider whether these observations of apparently old accretors could be explained by invoking Bondi-Hoyle accretion to rebuild a new disc around these stars during passage through a clumpy molecular cloud. We combine a simple Monte Carlo model to explore the capture of mass by such systems with a viscous evolution model to infer the levels of accretion that would be observed. We find that a significant fraction of stars may capture enough material via the Bondi-Hoyle mechanism to rebuild a disc of mass ≳1 minimum-mass solar nebula, and ≲10% accrete at observable levels at any given time. A significant fraction of the observed old accretors may be explained with our proposed mechanism. Such accretion may provide a chance for a second epoch of planet formation, and have unpredictable consequences for planetary evolution. Key words: accretion, accretion disks / protoplanetary disks / circumstellar matter / stars: formation / stars: premain sequence © ESO, 2014 1. Introduction Circumstellar discs form around protostars as a result of angular momentum conservation during gravitational collapse (e.g. Shu et al. 1987). In the early phases of star formation, disc material loses angular momentum and is accreted onto the central star. The most direct observational signature of the presence of a protoplanetary disc is the excess emission, on top of the expected naked stellar photosphere, at infrared and millimetre wavelengths, in the ultraviolet and in optical/infrared emission lines. The long wavelength emission is produced by a dusty disc, heated by internal dissipation processes or reprocessing of stellar radiation (e.g. Dullemond et al. 2007). The short wavelength excess and the optical/infrared emission lines are thought to be produced by the disc-star interaction as matter accretes onto the star or is ejected in a wind/jet (Hartmann 2009). Strong observational evidence shows that both the inner dusty disc and accretion onto the central star quickly disappear during the early stages of pre-main-sequence evolution; the fractions of stars with near infrared excess and with accretion signatures decay with an e-folding time of 2−3 Myr (Fedele et al. 2010; Hernández et al. 2007). This disc dissipation timescale, even considering the possible revision by Bell et al. (2013), sets a stringent constraint on the timescales for planet formation. Recent work has challenged this paradigm. Sensitive, wide field Hα surveys of large star-forming complexes in the Magellanic Clouds and our own Galaxy have revealed a population of pre-main-sequence stars that appear to be older than 10 Myr but still show prominent Hα emission and/or infrared excess (Beccari et al. 2010; De Marchi et al. 2013a,b, 2011a,c). Although some of these “old” accretor candidates in nearby star-forming regions have been shown to be misclassified young stellar objects (Manara et al. 2013), it is difficult to believe that this is the case for all the candidates; these populations of old accretors are not as centrally condensed as the young stellar clusters in the same fields (e.g. De Marchi et al. 2011b). If the line emission is interpreted as due to accretion as in young pre-main-sequence stars, the implied accretion rates are similar to those derived at early ages, and typically higher than nearby transitional discs 1. These findings are hard to understand in a framework in which the primordial disc is still the reservoir of accreting material at such old ages; even one disc of age >30 Myr implies an initial population >10 5 (assuming exponential decay with an e-folding timescale of 3 Myr). In this paper we explore the possibility that the old accretors do not have a primordial disc, but a disc that they re-accreted after the primordial disc had dissipated. Previous studies (Moeckel & Throop 2009; Padoan et al. 2005; Throop & Bally 2008) have investigated the influence of Bondi-Hoyle accretion on pre-main-sequence mass-accretion rates and the protoplanetary disc at earlier phases, during the initial evolution of the disc-star system within the progenitor cloud. Here we investigate the possibility that a star older than 5−10 Myr happens to travel through a clumpy molecular cloud, typically unrelated to that in which the star formed, and is able to accrete enough material to form a new accretion disc. 2. Modelling 2.1. Bondi-Hoyle accretion Hoyle & Lyttleton (1939), Bondi & Hoyle (1944), and Bondi (1952) proposed a mechanism by which objects can capture matter from the interstellar medium (ISM). A massive object moving through the ISM causes a perturbation, pulling material toward the object. As the capture of material is roughly symmetrical with respect to the direction of motion of the star, much of the angular momentum of the material cancels out, and hence it is captured by the star to eventually be accreted (Davies & Pringle 1980). The rate at which material is captured is given by (1) where v is the relative velocity between the star and the ISM, n is the number density of the ISM, and μ is the mean molecular weight (usually taken as 2.3mH). The gravitational cross-section is given by , where RBH is the Bondi-Hoyle radius the sound-speed of the ISM, typically 0.3kms-1. For a 1 M⊙ star moving at 1 km s-1, RBH ~ 1500 au. Parameters Values Parameters Values Fv 10-2, 10-3, 10-4, 10-5 Cs 0.3 km s-1 Nstars σ 105, 105, 106, 107 1 km s-1 Rd α 0.1 pc 2.35 4 -3 nd 10 cm (2)cs is Parameters for Monte Carlo models. To explore the effect of this process in reconstituting discs around young stars, we build a simple Monte Carlo model to treat interactions between stars and clumps with densities typical for molecular clouds. We assume a stationary clumpy molecular cloud, which we model as a collection of identical spherical clumps with radius Rcl and density ncl. We parametrise the density of clumps through a volume filling factor of dense gas fV. We assume a population of “old” young stars that has lost their primordial disc enters the cloud and moves through the clumpy medium. By randomly generating stars with masses between 0.7 M⊙ and 3.2 M⊙2 from a Salpeter IMF (M ∝ M− αSalpeter 1955) and velocities generated assuming a velocity dispersion of σv = 1kms-1, we sample the parameters required in Eq. (2) from the values given in Table 1. The model simulates 10 Myr treated as a series of quasi-static time steps of length tst = 2Rcl/v∗, assuming that each star is independent. For each star, we calculate RBH, the volume swept out per time-step , and hence the probability of encountering a dense clump (3)In each time-step a uniform random number ζ is drawn, and the star encounters a clump when ζ ≤ p; the impact parameter b of the encounter is given by drawing a second random number ζ2 from the same generator such that . We then determine the accretion rate (Eq. (1)) and resolve the stellar accretion and the clump-mass depletion on a finer time-grid of 1000 substeps to accurately determine the accreted mass. Interactions where RBH>Rcl and grazing encounters are treated correctly by taking the projected area of intersection. By repeating this process for >10 5 stars we build up meaningful statistics about the range of possible BH accretion histories and their probabilities. Note that each star is modelled independently, and mass accreted by a star does not influence the mass-budget available to later stars. The accretion histories determined by this model are then passed to a viscous evolution model (Sect. 2.2) to estimate the rate at which material is accreted by the star. Our choice of fV is based on a reanalysis of SPH simulations of star-forming regions including feedback mechanisms presented in Dale et al. (2012, 2013) to determine the filling factor of gas at densities higher than 10 4cm-3. We find that for bound clouds of similar stellar mass to the regions observed by Beccari et al. (2010); De Marchi et al. (2013b), 10-6<fV ≲ 10-3 irrespective of whether feedback from massive stars is included. While this provides a useful estimate of the amount of mass captured in this way, it somewhat overestimates the total as we neglect a number of physical processes. First, we neglect the motion of the clumps and assume that v = v∗ in Eq. (2). Correct treatment of the relative motions would in general reduce RBH and hence the accretion rates. Second, stars above 2 M⊙ have significant wind and radiation pressure that will depress the accretion rate (Edgar & Clarke 2004). Similarly, we do not include the possible influence of the X-ray photoevaporation on the accretion, which may have an analagous effect for lower mass stars. We also ignore the possible influence of magnetic fields, which recent studies (e.g. Lee et al. 2014) have shown may reduce accretion rates by a factor of a few. Likewise, we neglect structure on scales smaller than a single clump; such structure is required for a disc to form, and would reduce accretion rates relative to the homogeneous clump case treated here. Finally, we do not include binaries. However, the only influence of binarity in the context of Bondi-Hoyle accretion is to increase RBH, since binaries behave as a single object of mass M = M1 + M2. Fig. 1 Cumulative fraction of the stellar population that has accreted mass as a function of total accreted mass. The solid blue line indicates a filling-factor of 10-2, the dotted magenta line 10-3, the dashed red line 10-4, and the dot-dashed green line 10-5. 2.2. Viscous evolution modelling Due to the angular momentum of the material accreted from the clump, which may be due to a density gradient within the clump or the rotation of the clump itself, accretion cannot proceed directly onto the star (Ruffert 1997). Therefore, the formation of a thin accretion disc is expected as the result of the viscous spreading of a thin ring. Throop & Bally (2008) described the “buffer” effect of an accretion disc, but did not directly model it. We assume that the material accreted from the medium circularises at a radius r0 = 0.1RBH. After a single impulse of accretion onto the disc, the surface density is described by Σ(r) = M0/ (2π)δ(r − r0), where M0 is the deposited mass. Under the influence of an effective viscosity ν that redistributes the angular momentum in the disc, the spreading ring solution (Lynden-Bell & Pringle 1974) describes the evolution in time of this initial surface density, (4)where ν is the kinematic viscosity of the gas, Ω the Keplerian angular speed, I1/2 the modified Bessel function of order 1/2, λ = 2r3/2/ (3(GM∗)3/2νtr0), and we have specialized the expression for the ν ∝ r case. From this analytical solution, it is possible to compute the mass accretion rate onto the star Ṁkernel. To derive the mass accretion rate history onto the star, we convolve this function with the mass accretion rate history onto the disc: (5)Given a stellar mass, the loading radius, and a law for viscosity, the evolution in time is now completely determined. We fix the viscosity by using the well-known Shakura & Sunyaev (1973) prescription, ν = α(h/r)2r2Ω, where α is the Shakura-Sunyaev parameter and h/r the aspect ratio of the disc. We choose typical values of α = 0.01 and h/r = 0.05(r/ 1 AU)1/4 (Armitage 2011). Operationally, we sample Eq. (4) numerically on a space and time grid. We integrate over space to get the mass of the disc and we numerically differentiate the result to get the mass accretion rate kernel, which can be convolved with the Bondi-Hoyle history (Sect. 2.1). Fig. 2 Fraction of the population that would be detected as an old accretor at a given time, plotted as a function of the instantaneous accretion rate. The models are indicated using the same colours and line-styles as Fig. 1. 3. Results Our model indicates that a fraction of the population ~ 40−50 × fV encounter dense regions and accrete more than 0.001M⊙ material by the end of the simulation (Fig. 1). The median accreted mass is typically ~0.01 M⊙, similar to the mass of discs around young pre-main-sequence stars, with strong dependence on the stellar mass. In extreme cases, however, more massive stars (>2 M⊙) with low v∗ that encounter several clumps can capture ≥M⊙. Our treatment of the disc formation and evolution is probably inadequate for these extreme cases. Converting the Bondi-Hoyle accretion into stellar accretion rates, we find Ṁ∗ ≲ 10-6M⊙yr-1 after the formation of the disc. Owing to the assumptions inherent in our model, this rate declines from the peak as a power law as in primordial discs. By calculating the time each star spends accreting above a certain threshold accretion rate, one can derive a mean time per star as a function of the threshold and hence an estimate of the fraction of the population which one expects to observe accreting at a given time. As shown in Fig. 2, for a threshold rate of 10-8M⊙ yr-1 we typically find that the cumulative probability is ~20fV, i.e. the fraction of a stellar population that one expects to observe as old accretors at a given time is an order of magnitude larger than the volume filling-factor of dense clumps. 4. Discussion Our primary goal is to assess whether the Bondi-Hoyle mechanism can contribute significantly to observations of old accretors in regions with ongoing star formation, under a number of simple assumptions. This involves stars from a previous star-formation episode, after their primordial discs have dispersed, interacting with a clumpy molecular cloud. Our model indicates that up to several percent of the population passing through a region containing dense clumps may accrete more than 0.001 M⊙ of material. Because of the factors indicated above (Sect. 2.1), the model is likely to overestimate the total accreted mass. However, since the Bondi-Hoyle accretion is a well-understood process, the largest sources of uncertainty derive from the parameters assumed as input to the model, and in particular the clump geometry and filling factor, as well as the assumption that the accreted material will form a thin disc. Our initial choice of filling factor was based on a reanalysis of the simulations of Dale et al. (2012, 2013) for clouds similar to those observed to host old accretors. A further estimate can be obtained from the high-resolution submm maps of the 30 Dor region from Indebetouw et al. (2013). These reveal a wealth of clumpy structures, similar in scale and density to the clumps in the Monte Carlo model used here. Assuming that the clumps are uniform spheres with an average radius Rcl = 0.15pc and distributed in a cube whose depth is equal to the projected size of the observed region (10 × 10 × 10pc3) yields a filling factor of fV = 1.5 × 10-3, at the upper end of our parameter range. The behaviour of the accretion disc depends strongly on the viscous timescale τν, as parametrised in terms of r0 and α. An order of magnitude change in τν has little effect on the observable old-accretor fractions at low thresholds, but the fractions at high thresholds decline approximately in proportion to 1 /τν. For larger changes in viscosity, this also affects the lowest thresholds explored in Sect. 3. Since we do not include stars down to the peak of IMF (~0.3 M⊙) and Bondi-Hoyle accretion rates are ∝M2, we may overestimate the total fraction of old accretors by a factor ~3 for the Salpeter IMF assumed here. However, Eq. (3) is dominated by Rcl for low-mass stars, so one would expect a similar fraction of old accretors when Ṁ is a factor of 4 lower. Comparisons between our model and the observations of old accretors are difficult, as there are no firm constraints on the size of the old population (including non-accretors). Nevertheless, from Fig. 2 one can see that without an unrealistically large filling factor (≫10-3) of dense clumps, the small, nearby star-forming regions are unlikely to produce more than one old accretor, as their typical mass is a few hundred M⊙. As no old accretors have been identified in these regions, this is consistent with our model. From the recent identification of a large (~3 × 10 3M⊙) diffuse population with ages ≳10 Myr toward Orion (Bouy et al. 2014) one expects a few tens of reformed discs, although it is unclear whether there is any overlap between this population and the Orion molecular clouds. Observations of old accretors in large star-forming complexes typically detect up to several hundred such sources in each observed region. Given the formation efficiency we have computed and our assumed filling factors, this requires a total population at least of the order of 10 4 stars in the mass range of the observed old accretors, or ~3 × 10 4 stars correcting for the IMF, which must have passed through the regions in which the clumps are distributed. In the case of NGC 3603, which is inferred to have a population ~104.2M⊙ (Rahman et al. 2013) and ~100 old accretors, this implies either that the old population was significantly richer, or that fV is or was very high. The 30 Doradus region, on the other hand, shows a similar total of old accretors, although the total population is likely ~100 times larger than NGC 3603. Only a small fraction (1%) of the stars in 30Dor need to pass through regions containing dense clumps to produce the observed numbers. In reality, fV will evolve with time, and it is possible that the difference we observe between these regions may be due to 30Dor being more evolved, or having evolved more rapidly, than NGC 3603. In our model, a significant fraction (up to several tens of percent) of stars capture enough material to form a circumstellar disc of mass similar to primordial protoplanetary discs. This raises a number of interesting questions, such as whether a second epoch of planet formation is possible, and how the interaction between inflowing material and an existing planetary system might alter the accretion or the planetary evolution. The answers to these queries depend strongly on how the inflowing material interacts with the existing system, which we have not treated. Nevertheless, Bondi-Hoyle accretion presents a mechanism by which a new reservoir of potentially planet-forming material may be built by up to a few percent of stars. This gives them a second chance to form planets, from material that is potentially of different composition from the material that formed the star. Another possibility is that these stars are already surrounded by a planetary system formed out of the primordial disc. If they accrete new material, typically with an angular momentum different from that of the original planetary system, the interaction of the new material and the existing planets may have a range of outcomes. Understanding the range of possible outcomes will require detailed simulations of the accretion process and of the dynamical interactions with the planetary systems which are beyond the scope of the present paper. 5. Conclusions We have presented a model in which Bondi-Hoyle accretion by stars passing through dense clumps in the outer regions of their natal molecular cloud leads to the re-formation of a circumstellar disc. As a result, these stars may masquerade as pre-main-sequence objects due to ongoing accretion and the presence of infrared excess emission. A significant part of the observed populations of old accretors in large star-forming regions may be explained by this mechanism. As it may have wide-ranging consequences for the early evolution of planetary systems in rich stellar environments, we believe that further investigation of this mechanism is warranted. 1 Although these are systematically lower mass objects. 2 Stars above ~3 M⊙ have strong winds which make a simple model inappropriate, while observations of old accretors are incomplete for stars below 0.7−1 M⊙ depending on the distance to the observed region. Acknowledgments We wish to thank the anonymous referee for her/his careful reading of the text. The idea explored in this paper came up during discussions at the ESO science days and star formation coffee as well as the Munich Star Formation workshops. We thank the ESO Office for Science and all the institutes in the Munich area for providing a stimulating environment. We thank P. Armitage, G. Beccari, G. Costigan, B. Ercolano, G. De Marchi, C. Manara, N. Moeckel, A. Natta, P. Padoan, R. Siebenmorgen and S. Wolf for discussions and insights on the various aspects discussed in this paper. P.S. is supported under DFG programme no. WO 857/10-1. G.R. acknowledges the support of the International Max Planck Research School (IMPRS). This research was supported by the DFG cluster of excellence “Origin and Structure of the Universe” (JED). References - Armitage, P. J. 2011, ARA&A, 49, 195 [NASA ADS] [CrossRef] (In the text) Beccari, G., Spezzi, L., De Marchi, G., et al. 2010, ApJ, 720, 1108 [NASA ADS] [CrossRef] (In the text) Bell, C. P. M., Naylor, T., Mayne, N. J., Jeffries, R. D., & Littlefair, S. 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Records de altas temperaturas, (35º C o más), han sido aproximadas o se han roto en: Lituania, Polonia, Belarus, Estonia, Latvia, y Suecia, a fines de Julio y comienzos de agosto. Las altas temperaturas han secado bosques y creado incendios de vegetación en Siberia; en los estados de Oregón, Washington, y California, USA; y en las provincias de British Columbia, Alberta y Territorios del Noroeste de Canadá. Al mismo tiempo, aire frío llegado de altas latitudes sobre casi todo USA ha causado records de bajas temperaturas diurnas y nocturnas para la época en lugares tan al Sur como Florida y Georgia. Las temperaturas alcanzaron las de niveles de invierno en las montañas de Tennessee. Los mapas muestran las anomalías en temperaturas superficiales entre Julio 27 y Agosto 03 de 2014. Se hicieron con los datos colectados por el satélite MODIS, y comparados con los datos del mismo período entre 2005 y 2013. La observación de temperaturas por satélites alrededor del planeta se hace por la cantidad de radiación infrarroja emitida por la superficie terrestre, calentada por la radiación solar en el día y enfriada durante la noche. Estas no son temperaturas absolutas, se refieren a la temperatura del suelo al tocarse. Ellas muestran cuanta temperatura esta fuera del promedio. Los colores rojos intensos muestran temperaturas de hasta 10º C por encima del promedio, los azules cuanto más frío. Las áreas grises son zonas donde los datos están incompletos, o regiones cubiertas por nubes donde no se pudo medir la temperatura durante el período de observación. Los meteorólogos ven varias posibles causas y relaciones para las ondas de calor y patrones de enfriamiento. Sistemas de alta presión sobre Escandinavia y Norte de Rusia, así como en el Pacífico Noroeste de Norteamérica, permitiendo que masas de aire estable construyan domos calientes que bloqueen los frentes de aire que pueden traer cambios en vientos, precipitación, y temperaturas. Estos patrones de bloqueo causan juntos inusuales curvaturas y serpenteos en la corriente de chorro, la cual toma un patrón de dirección Norte – Sur, en el hemisferio Norte. La corriente de Jet mueve aire del Pacífico al Norte y calienta el Noroeste de Canadá y Costa Oeste de USA; va al Sur desde áreas frías de Canadá al centro y Este de USA; y lleva aire cálido del Atlántico hacia el Norte europeo. El Jet toma un Zigzag similar en el Oeste y Este de Siberia. Este patrón es mucho más común en Invierno que en Verano. References and Related Reading Accuweather (2014, August 5) Heat Replaced by Storms in Central, Eastern Europe. Accessed August 7, 2014. Accuweather (2014, July 29) Hot July for Much of Europe. Accessed August 7, 2014. The Guardian (2014, July 17) Is global warming causing extreme weather via jet stream waves. Accessed August 7, 2014. NASA Earth Observatory (2014, January 10) What Goes Around Comes Around. NASA Goddard Institute for Space Studies (2012, August) The New Climate Dice: Public Perception of Climate Change. Accessed August 7, 2014. Weather.com (2014, August 1) July Cooldown Part Two: Polar Plunge Return. Accessed August 7, 2014. Weather.com (2014, July 20) July Chill Brought Record Cold Temperatures. Accessed August 7, 2014. Weather Extremes Blog, via Weather Underground (2014, August 5) First 100°F Temperature on Record in the Baltics. Accessed August 7, 2014. NASA Earth Observatory images by Jesse Allen, using data from the Land Processes Distributed Active Archive Center (LPDAAC). Caption by Michael Carlowicz, with image interpretation from Bill Patzert (NASA JPL), Jason Samenow (The Washington Post) and Linus Magnusson (European Centre for Medium-Range Weather Forecasting). Instrument(s): Terra - MODIS Deflexión de la luz por el Sol. Carlos Gil, ACA Las teorías de la deflexión de la luz por un cuerpo masivo, provienen desde mediado del siglo XVII, cuando el reverendo John Michell, un clérigo inglés y filósofo natural, razono de que si el sol fuera lo suficiente masivo, la luz no podría escapar de su superficie. El pionero de la descripción matemática de la gravedad, Sir Isaac Newton, aparentemente no escribió nada acerca de los efectos que un cuerpo masivo tendrían sobre la luz, pero existe una nota en su tratado de óptica publicado en 1.704, sobre las partículas de luz, las cuales podrían ser afectadas por la gravedad, de la misma manera que ocurre con la materia ordinaria. El primer cálculo sobre la deflexión de la luz por un cuerpo masivo fue publicado por el astrónomo alemán Johann Georg von Soldner en 1.801. Soldner demostró que los rayos de luz de una estrella ubicada a una distancia similar a la del sol, podrían reflectar la luz por un ángulo de acerca 0.90 segundos de arcos, o un cuarto de milésima de un grado. Este ángulo corresponde a un diámetro aparente de un disco compacto (CD) visto desde una distancia de cerca 30 kilómetros (aproximadamente20 millas) Los cálculos de Soldner fueron basados en las leyes del movimiento y gravitación de Newton, asumiendo que la luz estaba constituida por partículas que movían rápidamente. Como sabemos, ni Soldener o astrónomos posteriores intentaron verificar esta predicción, por una buena razón, realizar este experimento estaba más allá de la capacidad de los instrumentos astronómicos en los inicios del siglo XIX. Deflexión de la luz en teoría general de la relatividad Un siglo después, en los inicios del siglo XX, Einstein desarrolla su teoría general de la relatividad. Einstein calculo que la deflexión estimada por su teoría seria dos veces el valor establecido por la teoría de Newton. Figura No. 1 La figuraNo.1, muestra la deflexión de los rayos de luz que pasan cerca de una masa esférica. Para hacer visible este efecto, esta masa fue calculada haciéndola igual a la del sol, pero teniendo un diámetro, cincuenta mil millones de vece menor. De acuerdo a la teoría general de la relatividad, un rayo de luz aproximándose a un cuerpo masivo, tal como el sol, desde su origen. Su trayectoria es deflectada tal como se observa en la figura No. 2. El valor del ángulo deflectado, es inversamente proporcional a la distancia (Ro), del centro de masa. Figura No. 2 La teoría general de la relatividad ofrece la siguiente ecuación general para la trayectoria de un rayo de luz, afectado por la presencia de un cuerpo masivo tal como el sol Las raíces de esta ecuación de segundo grado, ubican los valores del ángulo Ø1, en el segundo y tercer cuadrante como se observa en la figura No.2 y cuyos valores son( )y ), correspondiendo un valor total del ángulo de deflexión de: Cuando G, M y C toman los valores de: , δtoma el valor de: δ = [radianes] = 1,77 [segundos de arco] La comprobación observacional de este valor, se realizó en la expedición efectuada en 1.919, organizada y conducida por Eddigton, al visitar las islas Prince, ubicadas cerca del África, para presenciar y fotografiar un eclipse total de sol, así como también tomar medidas de las estrellas alrededor del sol durante el eclipse, al respecto de este resultado existe la siguiente anécdota: Eddigton le comento a Einstein los resultados obtenidos sobre esta medición, y este simplemente dijo “– Lo sé, la teoría es correcta - “. ¿Y si no se hubiese deflectado? Einstein: le respondió a Eddington, “Pues lo hubiera sentido por el buen Dios. La teoría es correcta”. Tres años antes de esta expedición, en una carta dirigida a ArnoldSommerfeld, Einstein escribía: “Usted se convencerá de la Relatividad General una vez la haya estudiado. Por consiguiente, no voy a decir una palabra en su defensa”. Figura No. 3 La figura No 3, muestra fotografía tomada con el telescopio Kepler, de un sistema binario, en el que se puede apreciar perfectamente como la luz se curva a causa de la gravedad. Bibliografía.Mathematical Physics by Donald H. Menzel.- Dover Publications, Inc. New York – 1.961 Introduction to Relativity by H. A. Atwater – Pergamon Press, Oxford -1.974 A short course in General Relativity – J Foster and J. D. Nightingale – Springer - New York – 1.994. Nota del autor.-La fotografía mostrada como la figura No. 3, ha sido tomadadel artículo “La expedición de Eddigthon, Einstein tenía razón” www.medciencia.com Born between November 29 and December 18? Here’s your constellation Born somewhere between November 29 and December 18? If so, chances are the sun passes in front of the constellation Ophiuchus the Serpent Bearer on your birthday. Now I can almost hear someone saying: Wait a minute! There’s no Ophiuchus on the horoscope page. You are absolutely correct. That’s because Ophiuchus is a constellation – not a sign – of the Zodiac. Follow the links below to learn more about astrological signs versus astronomical constellations, when and where to locate Ophiuchus, some deep-sky treasures it contains, its mythology, its science and more. On a dark, moonless night, look for Ophichus above the bright ruddy star Antares. Image via Till Grednar. Astrological signs versus astronomical constellations. The sun is in the sign Sagittarius from November 21 to December 21. But, in the present-day sky, the sun is in front of the astronomical constellation Ophiuchus from about November 29 to December 18. In 2014, the sun enters the constellation Ophiuchus on November 30 at 7:00 Universal Time (or for the U.S. Central Time Zone: November 30, at 1:00 a.m. CST). Then the sun enters the constellation Sagittarius on December 18, 2014, at 13:00 Universal Time or 7:00 a.m. CST. Whether you’re speaking about astrological signs or astronomical constellations, the Zodiac depicts the narrow beltway of stars on the stellar sphere through which the sun, moon and planets travel continuously. The Zodiac runs astride the ecliptic – the sun’s yearly pathway in front of the backdrop stars. The band of the Zodiac extends some 8o north and south of the ecliptic, spanning a total of 16 o in width. The sun is said to enter the sign Sagittarius around November 21, or whenever the sun is precisely 30o west of the December solstice point. The sun then enters the sign Capricorn on the December 21 solstice. So the sun passes through the sign Sagittarius for the month period before and up to the December solstice, irrespective of the sun shining in front of the constellation Ophiuchus from November 29 to December 18. By the way, the December solstice point moves one degree westward in front of the zodiacal constellations – or backdrop stars – in about 72 years. The December solstice point will finally move into the constellation Ophiuchus by the year 2269. When and where to locate Ophiuchus. The best time to observe Ophiuchus is during a Northern Hemisphere summer or a Southern Hemisphere winter. From the Northern Hemisphere, late July and early August present this constellation high in the southern sky at nightfall and early evening. It’s seen in the southwest sky on autumn evenings in the Northern Hemisphere. This rather large constellation fills the area of sky to the north of the constellation Scorpius the Scorpion and to the south of the constellation Hercules the Hero. If you’re familiar with Scorpius’ brightest star Antares, try star-hopping to Ophiuchus from this ruddy gem of a star. The head of Ophiuchus is marked by the star Rasalhague (Alpha Ophiuchi). Ophiuchus is joined in legend and in the sky to the constellation of the Serpent. If you have a dark sky, you might find this is one constellation that looks like what it’s supposed to be: a big guy holding a snake. The name Ophiuchus comes from two Greek words meaning serpent and holding. Can you see the Pipe Nebula a little to the upper right of center? If not Ophiuchus the Serpent Bearer. Deep-sky objects in Ophiuchus. On a night when the moon is absent, take your binoculars and use them to scan Ophiuchus, which lies near the band of the Milky Way and so has many deep-sky wonders. Ophiuchus boasts of numerous globular clusters, for example. The two easiest globular clusters to see with ordinary binoculars are M10 and M12, as shown on the above chart. Through binoculars, they look like faint puffs of light, but with the telescope, you begin to see these globular clusters for what they really are. They are immense stellar cities spanning a hundred to a few hundred light-years in diameter, teeming with hundreds of thousands of stars. Another big deep-sky favorite is the Pipe Nebula, a vast interstellar cloud of gas and dust sweeping across about 7o of sky. At an arm’s length, that’s about the width of three to four fingers. This dark nebula resides at a distance of 600 to 700 light-years in southern Ophiuchus, and can be seen with the unaided eye in a dark, transparent sky. The Pipe Nebula is found due east of the star Antares, and due north of the stars Shaula and Lesath. These two stars (but not the Pipe Nebula) are shown on the above chart. The Greek Asclepius or Latin Aesculapius. The constellation Ophiuchus represents this legendary physician. Ophiuchus in myth and star lore. In Greek sky lore, Ophiuchus represents Asclepius – said to have been the first doctor – always depicted holding a great serpent or snake. Depending on how it’s used, a snake’s venom can either kill or cure. It’s said that Asclepius concocted a potion from this snake venom, the blood of the Gorgon monster and an unknown herb to bring the dead back to life. This greatly alarmed the gods as it threatened to undo the natural order of things. As the good doctor was trying to bring Orion the Hunter back to life, the god of the Underworld pleaded to Zeus, the king of the gods, to reconsider the ramifications of the death of death. Apparently his argument swayed the king of the gods. Zeus confiscated the potion, removed Asclepius from Earth and placed the gifted physician into the starry heavens. We hardly know how the god of the Underworld made his appeal. Perhaps he said only that which never lives never dies, and that no mortal can have one without the other. The absence of death means the absence – not the continuance – of life. Sophocles may have expressed the myth’s inherent message when saying: Better to die, and sleep the never-waking sleep, than linger on and dare to live when the soul’s life is gone. Possibly, the poet T.S. Eliot reechoed the theme of the ever-living story in his Four Quartets: We die with the Dying See they depart and w ego with them We are born with the dead: See, they return, and bring us with them. In any event, the association with Asclepius with snakes is why we sometimes see a staff with a serpent wound around it at doctor’s offices and hospitals, even today. The great Johannes Kepler (1571 to 1630). The star known as Kepler’s supernova exploded in 1604, within the boundaries of the constellation Ophiuchus. Ophiuchus in history and science. It’s been more than 400 years since anyone has seen a supernova explosion of a star within our own Milky Way galaxy. But in the year 1604, a supernova known as Kepler’s Supernova exploded onto the scene, attaining naked-eye visibility for 18 months. It shone in southern Ophiuchus, not all that far from the Pipe Nebula. Kepler’s Supernova in 1604 came upon the heels of Tycho’s Supernova that lit up Cassiopeia in 1572. These supernovae sent shock waves into the intelligentsia of Europe, which firmly believed in the Aristotelian notion of an immutable universe outside the orbit of the moon. Tycho Brahe took a parallax measurement of the 1572 supernova, proving that it could not be an atmospheric phenomenon. In fact, the supernova shone well beyond the moon’s orbit. Shortly thereafter Kepler’s Supernova in 1604 seemed to drive home the point all over again. Moreover, Tycho Brahe measured the distance of a comet in 1577, also finding it to be farther away than the moon. Aristotelians wanted to believe comets were gases burning in the atmosphere, but once again, Tycho threw cold water on the idea of Aristotle’s immutable universe. What else can we tell you about Ophiuchus? Only that it lies in the direction to Barnard’s Star, which has caused a gleam in the eye of many an earthly dreamer. This relatively nearby star – only about six light-years away – was the center of a controversy about possible planets during the decade from 1963 to about 1973. Many astronomers accepted a claim by Peter van de Kamp that he had detected, by using astrometry, a perturbation in the proper motion of Barnard’s Star consistent with its having one or more planets comparable in mass with Jupiter. Ultimately, that claim was refuted, and to date no planet has been found for Barnard’s Star – nor are any expected. Barnard’s Star, located in the direction to the constellation Ophiuchus. Our corner of the universe got a little lonelier when astronomers determined in 2012 that that Barnard’s Star – which is only six lightyears away – has no planets of Earth’s size or larger in its habitable zone. Bottom line: The sun lies within the boundaries of the constellation Ophiuchus the Serpent Bearer for about two weeks of every year, and thus Ophiuchus is an informal member of the Zodiac. Astrological signs versus astronomical constellations, how to locate Ophiuchus, some deep-sky treasures it contains, plus charts and more. Kepler 62e y 62f Planetas Acuosos "Estos planetas no se parecen a nada en nuestro sistema solar. Están cubiertos con océanos infinitos", dijo Lisa Kaltenegger, del Instituto de Astronomía Max Planck, que estudió los planetas. Se trata de los dos planetas de la estrella Kepler-62, que se encuentra a 1200 años luz de la Tierra, en la constelación de Lira. Dos de sus cinco planetas, llamados Kepler-62e y Kepler-62f, están en la zona habitable de la estrella, es decir, están a una distancia de su sol que les permite mantener la temperatura necesaria para que exista el agua en estado Líquido lo que es imprescindible para la aparición de la vida. En estos planetas hay agua y mucha. La vida podría existir, por tanto, pero no se sabe si podría existir alguna civilización. "La vida en estos planetas debería sobrevivir debajo del agua, lo que hace difícil conseguir los metales, desarrollar la metalurgia y crear la electricidad requeridos para la existencia de una civilización", señala Kaltnegger. "Sin embargo, los mundos podrían tener una gran belleza, con un océano azul bajo un sol de color naranja. Y quién sabe, quizá podría existir vida lo suficientemente inteligente para desarrollar tecnología hasta un nivel que nos sorprendería", añade Kaltnegger. Meteorito en Nicaragua? septiembre 7, 2014 Posteado por Julio Vannini en Actualidad Astronómica Eran las 11:04 pm del Sábado 6 de Septiembre del 2014. Me encontraba procesando unas fotos para mis álbumes en Flickr cuando de repente las redes sociales en Nicaragua literalmente estallan. Comentarios alarmados tanto en Facebook como en Twitter de un tremendo sonido semejante a una explosión que se hizo sentir en gran parte de la ciudad capital, Managua y con repercusiones sísmicas también. Por espacio de dos horas di seguimiento a las redes sociales en donde todo tipo de especulación salió a flote. No era de extrañarse ya que un estruendo así de fuerte según los reportes, puso en vilo a casi toda Managua. Una de las hipótesis que empezó a sonar con fuerza fue que un meteorito había caído sobre Managua. Varios amigos empezaron a consultarme sobre el tema. Al respecto debo aclarar, que a falta de evidencia solida, cualquier cosas que se diga no puede ser considerada como algo concreto, hasta que dicha evidencia saliera a la luz. A las consultas hechas exteriorice mi opinión inicial: un meteorito que hubiese generado semejante estallido debió notarse en el cielo. Como no había ningún avance en el termino noticioso, decidí dormirme y esperar la postura oficial la cual se hizo pública después del mediodía de hoy (Domingo 7 de Septiembre). En resumen: se encontró un cráter de 12 metros de diámetro por unos 5 de profundidad en los terrenos de la Fuerza Aérea de Nicaragua. El experto de INETER que se presento en la televisión dio a conocer que la versión oficial de los hechos fue la caída de un meteorito, basado en: 1. El tipo de estallido. 2. Registros en sismógrafos: uno del estallido inicial y otro al momento del impacto. 3. Sus memorias de un evento “similar” ocurrido hace tiempo atrás. 4. El cráter. Foto tomada del sitio web de periódico Hoy, de Managua. Como seguramente habrán notado, el meteorito resultante de la caída en Peekskill fue lo suficientemente grande para sobrevivir su paso ardiente por la atmósfera y caer a tierra sin necesidad de crear un cráter. La gran mayoría de rocas que se encuentran de tamaños similares no han creado cráter alguno y se encuentran simplemente a flor de suelo. Como habrán notado también, el paso de esa roca fue bastante, bastante llamativa. No fue algo de un resplandor sino un evento que fue rastreable por muchos segundos en el cielo. En lo personal he presenciado un par de bólidos que se han quemado sobre la tierra, siendo el más notable el avistado el 14 de Octubre del 2013 cuando iba rumbo a Granada. Un recuento en Twitter del avistamiento. Un cráter de ese tamaño (12 metros de diámetro y 5.5 metros de profundidad) debió ser causado por un objeto con suficiente mesa y energía cinética. Aunque es difícil precisarlo así al aire, uno puede pensar que el meteorito resultante debe ser bastante grande. Personalmente mantengo contacto el Dr. Plait quien por correo me ha sugerido un cuerpo de al menos un metro de diámetro como el causante de ese cráter. En otras palabras: pedazo de roca espacial que se encuentra ahí enterrada! Pero, realmente es eso lo que ocasiono el estruendo y el cráter mostrado a los medios? Antes de continuar quiero dejar bien en claro que lo siguiente es mi opinión personal, basada en los modestos conocimientos de astronomía que poseo y los estudios realizados sobre cráteres de impacto y comentarios con otros astrónomos. Soy un astrónomo profesional titulado como tal? No, solo soy un astrónomo amateur que le gusta mucho investigar y tratar de encontrar la verdad de las cosas por medio de evidencias. No es de mi interés especular sobre qué fue lo que paso anoche. Lo que planteare son las razones por las cuales no creo que sea un meteorito y porque no me convence lo anunciado públicamente. Ustedes tienen todo el derecho de llevarme la contraria en esto si así lo desean. Ok. 1. Un meteoro lo suficientemente grande para dejar un cráter así debió ser visto por mucho tiempo en el cielo. Sábado por la noche donde miles de capitalinos se encuentran afuera de sus casas y nadie vio nada salvo un resplandor a modo de estallido en la zona de carretera norte. Pongo en referencia el video de Plait. No existe tal cosa como un “meteoro sin estela”. Me atrevo a agregar a esto que no se mostrado video alguno del evento (hasta el momento) que pudiese haber sido captado por cámaras de seguridad instaladas en Managua. 2. Reporte de resplandor y no de bola de fuego. 3. Según el reporte oficial se registraron dos eventos sísmicos en los sismógrafos de INETER, uno del estallido y otro del golpe en tierra. Mi pregunta: Como saben que esos registros se deben a eso? Con que otro dato hacen la correlación? Si no hay video ni registro visual del evento, como saben a ciencia cierta que esos eventos sísmicos provienen de un meteorito cayendo? Es decir: pudo ser cualquier otra cosa. 4. Si encontraron el cráter. Por qué no han excavado? Un meteorito no es algo peligroso una vez en tierra. Es algo que se puede tocar inmediatamente. A diferencia de la creencia popular, no necesariamente debe haber indicio de fuego o cosas quemadas, eso es puro cine. Cuando un meteorito cae, este ha sido frenado tanto por la atmósfera que viene frió. Eso de estar esperando ayuda internacional para investigar realmente no es necesario. Estamos hablando del Ejercito Nacional. No costaba nada llegar y excavar para sacar lo que haya estado ahí. Además, tienen expertos en bombas y personal científico que los atiende y ninguno de ellos pudo saber cómo desenterrar un meteorito? Hasta yo que soy un amateur sé lo que debo de hacer! Ahí adentro debería de haber una roca bien grande esperando ser rescatada. Dice una formula muy conocida en el campo de la Física: Fuerza = Masa x Aceleración (F = m.a). Si la aceleración es tal que hace que la velocidad sea terminal, entonces la masa debe ser muy grande para liberar la Energía (Fuerza de impacto) suficiente para ese cráter. Además, es terreno blando. ESA PIEDRA DEBE ESTAR AHI! 5. Meteoritos de hielo. De donde sacaron eso? Para saber cuáles son los tipos de meteoritos existentes, les dejo el siguiente enlace. Wikipedia: Tipos de meteoritos. Yo personalmente poseo algunas muestra pequeñas y he tenido la oportunidad de estudiar otros más grandes en Boston, Massachussets. Por cierto, cuando traje esas muestras, el personal de Aduanas me pidió que consiguiera certificación de algún tipo con el MAGFOR para asegurarse que no habría peligro de contaminación extraterrestre. (Muchos Hombres de Negro o Evolución, por lo visto) Y bien, estas son las razones por las cuales yo personalmente creo que lo de anoche no fue un meteorito sino otro evento. ¿Que evento fue ese? No tengo la más mínima idea y como dije, no quiero especular al respecto. Pero si fue otra cosa, considero como ciudadano Nicaragüense que se nos debe respetar y hablar con la verdad. Ahora, si en realidad fue un meteorito y se muestra su extracción, análisis y composición, con gusto daré por cancelada mi postura. Realmente estaría muy contento de que un evento poco probable como ese haya sucedido en Nicaragua (por la pequeña extensión territorial que poseemos) y sobre todo que no haya causado pérdidas de vidas. Estaría realmente feliz que se demostrara que estoy equivocado, pero no con palabras, sino con las evidencias reales de la extracción de ese supuesto meteorito. Mientras tanto, mis 5 pesos en la bolsa le van a que fue otra cosa. Y ustedes, que opinan? Imagen del Busto de Bolívar en el pico Bolívar desde la Hechicera a 15 Kilómetros de distancia con la Técnica de Lucky Imaging. Antonio Ballesteros Motín (Centro de Investigaciones de Astronomía, CIDA) Agosto 2014, e-mail : ballesteros @cida.gob.ve La técnica de Lucky Imaging (L.I.) que se utiliza en astrofotografía, consiste básicamente en tomar muchísimas imágenes, cientos o mejor miles con tiempos de exposición muy cortos del orden de milisegundos para congelar la turbulencia atmosférica en algunas tomas dependiendo del seeing del sitio y sumar las mejores imágenes de la serie. Esto se hace con programas como el AutoStakker y RegiStax. Los profesionales suman desde el uno al cinco por ciento de las mejores de la serie, porque tienen muchas imágenes que van desde 50.000 o máximo de un millón de cada objeto, mientras que los aficionados tienen cientos o miles, y suman entre el 10 y el 50 % de las mejores de la serie. En internet, el término “Lucky Imaging” se usa muy alegremente como, por ejemplo, “Imagen de la nebulosa de Orión utilizando Lucky Imaging” y cuando uno va a la página donde está la imagen y sus datos uno encuentra lo siguiente: se sumaron tres imágenes de 42 segundos c/u. Lo que quiere decir que simplemente es una suma de tres imágenes con un total de 126 segundos de exposición. En otra página conseguimos, El trapecio en Orión con (L.I.): imagen LRGB L: 300 x 1 seg. R,G,B: 150 x 1 seg. de cada color, que igualmente representa solo la suma de 750 imágenes para un total de 12,5 minutos de exposición. El poder de L.I. es la selección de las mejores imágenes, porque si uno suma todas las imágenes, las pocas enfocadas (atmosfera congelada) con las movidas o desenfocadas, que son la mayoría, resulta en una imagen borrosa. Hagan una prueba con el RegiStax, tomen un video de un detalle de la luna o un planeta y sumen primero todas, después 50% y el 10% de las mejores, y procesen las tres imágenes con los wavelet del RegiStax o PhotoShop (niveles, mascara de enfoque etc.) y compare las tres imágenes. Noten que los tiempos de exposición deben ser de milisegundos, ya que tiempos de 42 seg. o de 1 seg. por imagen esto no es L.I. La imagen que usaremos de referencia la vi en una página de internet, es una sola imagen tomada con una cámara digital desde el sector de la Hechicera, probablemente hecha con un telescopio de 2.000 milímetros con un barlow 2X. Me pregunté si esto se puede mejorar con la técnica de L.I. considerando varias ventajas: el objeto esta fijo, no necesito seguimiento con motor y se hace de día, además tengo varios telescopios y cámaras digitales y puedo hacer pruebas con diferentes equipos. Las desventajas son que el objeto esta en el horizonte donde hay mucha turbulencia, es de color negro, está a 15 kilómetros de distancia (ver calculo) y es de unos 80 centímetros de altura. Desde la ciudad de Mérida es solo ocasionalmente temprano en las mañanas que el pico Bolívar (4.978 metros de altura m.s.n.m.) está despejado y hay mucha nubosidad el resto del día. Objeto a fotografiar, es la estatua del Libertador Simón Bolívar, en el pico Bolívar, la imagen se vería desde atrás, vista desde la Hechicera, Mérida. Derecha: Imágenes tomadas de Internet. Imagen tomada de una página de internet que usaremos de referencia para compararlas con las nuevas, a la izquierda detalle con ampliación de la misma imagen. Los valores dados para la altura sobre el nivel del mar del pico Bolívar en internet van desde los 4978 hasta 5,007 metros, siendo el valor más común el de 4978, de tal manera que el cálculo nos da aproximadamente 15 Kilómetros de distancia al objeto. Imagen Nº1, es del lugar donde se tomaron las imágenes del artículo, la terraza del CIDA en el sector de la Hechicera (ver datos de la imagen en la tabla). En una ampliación de la misma imagen, no hay resolución suficiente para observar el objeto en el pico Bolívar. Imagen N2º de el pico Bolívar (ver datos de la imagen en la tabla). A la derecha, en un detalle ampliado de la misma imagen, se nota la necesidad de mayor acercamiento para obtener mejor resolución del objeto. Imagen Nº3 tomada con un telescopio, (ver datos de la imagen en la tabla). A la izquierda en detalle ampliado de la misma imagen se observa una mejora de la resolución respecto a la imagen de referencia. Imagen Nº4 fue tomada con un telescopio de 16 pulgadas de diámetro. En el foco primario se colocó una cámara de video modelo Guppy Pro 503C, a color, de 5 megapixeles, tiempo de exposición de cada cuadro es de 10 milisegundos. Noten que cada cuadro pesa 14 Mb y durante la mayor parte del tiempo del video el sistema guarda los cuadros en la memoria resultando en un archivo de video de 3,4 Gb. El video es lento de 2.8 FPS para un total de 87 segundos y 243 cuadros, capturado utilizando un programa el FireCapture V2.3. Con RegiStax se sumaron el 30% de los mejores cuadros y procesado con wavelet. En el momento de la toma, había nubes altas oscureciendo el sitio, pero se nota una mejora de la resolución. Esta cámara tiene ROI (región de interés) y se puede variar la resolución del CMOS, es una especie de zoom con pérdida de resolución, pero aumenta la velocidad del video. La imagen Nº5 es de 1000x1000 pixeles, 10 milisegundos por cuadro igual que el anterior, pero video más rápido 15 FPS y un campo menor resultando en un archivo de 2,52 Gb., 904 cuadros, y una duración de 58 segundos. El objeto se ve más cercano, sin embargo el resultado no fue satisfactorio ya que la turbulencia se nota mucho más en el video y la imagen resultante no se percibe mejora. La imagen Nº6, algunos campos con la cámara Guppy Pro. La imagen Nº 7 de 800x600 pixeles, se tomo después de varios meses de intentos para lograr el mejor video. Ese día se tomaron cinco videos y el último fue el mejor de todos, de 11,7 milisegundos por cuadro, duración 116 segundos, 3809 cuadros, 32 FPS y un archivo final de 5,2 Gb. Se utilizó RegiStax V.6 para la suma y el procesado con los wavelet y ajustes finales con PhotoShop, en este caso, se sumaron solo los mejores 50 cuadros (1,3% del total). En conclusión, es evidente que la técnica de L.I. funciona, pero la resolución de la imagen depende fuertemente de las condiciones climáticas del lugar, turbulencia atmosférica, iluminación del objeto, nubosidad, hora del día, contaminación atmosférica (humo), etc. Le agradezco a Johnny Cova por su ayuda y paciencia que tuvo durante varios meses que montamos un sin número de veces el equipo, para lograr el video final. Distancia Focal Equivalente Si tenemos una lente de 200 milímetros y le colocamos una cámara Nikon D700 que tiene el sensor de 35 mm, lo que se llama comúnmente Full Frame o cámara para lentes Fx y se toma una imagen cualquiera para calcular el aumento o el campo de la imagen, se utiliza para el cálculo la focal del lente que es de 200 mm, por que el factor de multiplicación es de uno pero si la imagen se tomó con una Nikon D80, que tiene un sensor más pequeño el factor de multiplicación es 1,5 y el lente tiene una focal equivalente de 300 mm. Si se coloca una cámara como la Guppy Pro con un sensor mucho más pequeño, el factor de multiplicación es de 6,08 en la máxima resolución de la cámara y el lente es de 200 x 6,08 = 1.216 mm. En la tabla esta el factor de multiplicación para diferentes sensores. La mayoría de los datos se consiguen en internet. Un error muy común en los datos de las imágenes de planetas publicadas en internet que dicen por ejemplo la distancia focal es de 6 metros con barlow 2x o 9 metros con barlow 3x con un telescopio de 3 metros de distancia focal pero eso es cierto solo si el sensor de la cámara es de formato de 35 milímetros que en la mayoría de los casos no lo es, porque la cámara que utilizaron tiene un sensor de menor tamaño y la distancia focal es mucho mayor de lo que dicen los datos.. Si coloco la cámara Guppy Pro en un telescopio Meade de 16 pulgadas de diámetro y una focal de 4064 milímetros véase tabla más abajo, calculada por mí, como es una hoja de Excel y es interactiva puedo cambiar la distancia focal o el tamaño del pixels, si coloco 2800 mm, que es el Celestron 11 pulgadas que tengo en Caracas, los valores cambian automáticamente en la tabla con una resolución de 800 x 600 pixeles (Véase tabla), con Meade 16 tengo una distancia equivalente de 80 metros !!! Pero si uso el Celestron 11 me da un distancia focal equivalente de 55 metros, estas distancias focales parecen grandes pero los aficionados para obtener imágenes de los planetas Marte, Júpiter, Saturno usan normalmente entre 40 metros o más. Si uno divide el área del sensor de 35 mm (864 mm2) entre el área del sensor de la Guppy Pro (25 mm2) nos da 36 veces más pequeña el área en la máxima resolución de 2588 x 1940, si utilizo una resolución de 800 x 600 pixeles el área se reduce a un mas a 372 veces respecto a un sensor de formato de 35 mm., full frame (36mm x 24mm). Referencia: SUMA DE IMÁGENES DIGITALES, partes I y II. El Mensajero Estelar, páginas 20 a la 28, año 37, Nº 68, Octubre-Diciembre 2013. La desaparición de los géiseres gigantes de una luna de Júpiter desconcierta a los científicos © NASA Los géiseres gigantes detectados en 2013 por el Telescopio Espacial Hubble de la NASA en Europa, una de las lunas de Júpiter, parecen haber desaparecido, algo que ha dejado desconcertados a los científicos. Los enormes chorros de vapor de agua se han escondido de la vista de los observadores de Europa, el más pequeño de los 4 satélites galileanos de Júpiter. Los géiseres detectados por Hubble en diciembre de 2013 en las imágenes del menor de los satélites joviales proporcionaban una oportunidad para el descubrimiento de vida extraterrestre en el Sistema Solar. Los investigadores sospechaban que el vapor salía de las grietas que se abrían en el hielo debido a cambios Provocados por las fuerzas de la Marea, cuando la Luna se alejaba de Júpiter. De momento, los científicos son incapaces de explicar la desaparición de los géiseres. Las observaciones posteriores del Hubble llevadas a cabo en enero y febrero de este año no mostraron signos de estas columnas de vapor de alturas de hasta 200 km. Según investigadores, los géiseres de Europa pueden ser esporádicos como los volcanes de la Tierra, a diferencia de los expulsiones de vapor más o menos constantes que tienen lugar en el polo sur de Encélado, una de las lunas de Saturno que también alberga un océano bajo la superficie. La NASA busca obtener más datos sobre las expulsiones de agua antes del inicio de la misión espacial que a mediados de la década de 2020 llevará a cabo la sonda Europa Clipper, que realizará múltiples vuelos sobre la luna helada de Júpiter. Lluvia de Estrellas de Las Geminíadas Por. Jesús H. Otero A. Este 13 de Diciembre ocurrirá la lluvia de estrellas más intensa e interesante del año y podrá verse desde todo el país. Las primeras noticias que mencionan esta lluvia de meteoros datan de los años 1860´s. La primera observación conocida fue realizada por R. P. Greg de Manchester, Inglaterra, en 1862, cuando notó el radiante en la constelación de Géminis. Casualmente B. V. Marsh and A. C. Twining, de USA, hizo el descubrimiento al mismo tiempo. Entre el 10 y el 12 de Diciembre. Por su parte Herschell las reportó entre Diciembre 12 y 13 de 1863. A partir de aquí empezaron a hacerse más numerosas y el radiante fue catalogado como un radiante activo. Normalmente los radiantes están relacionados a las órbitas de los cometas, pero no existe ningún cometa con esta órbita conocido, en cambio si un asteroide llamado 3200 Phaeton. Las lluvias de estrellas ocurren cuando nuestro planeta pasa a través del tubo de polvo que va dejando un cometa tras sucesivos pasos, al impactar con las finas partículas de polvo que han ido siendo arrojadas por el cometa, se produce un fenómeno luminoso que es conocido como meteoro, o estrella fugaz. Los asteroides no arrojan material, así que lo más probable es que 3200 Phaeton sea un cometa extinto después de numerosos pasos por el Perihelio. Su período es muy corto, apenas 1.65 años, lo que explica su rápida extinción El número de meteoros observados se ha ido incrementando con el paso de los años. Hacia 1900, el radiante producía unos 20 meteoros por hora, en los años 1950´s unos 65, en los 1980´s la taza horaria era de 85 meteoros por hora. Pero el incremento sigue. Este año, por características orbitales especiales, se espera que puedan observarse hasta 200 meteoros por hora. Será una lluvia de estrellas fabulosa, con muchos meteoros rápidos, brillantes, y de color azul y verde. Se espera que para el 2050 el radiante produzca unos 200 meteoros horarios, pero a partir de aquí irá declinando poco a poco, hasta desaparecer hacia el año 2100. Este 13 de Diciembre, si el firmamento se nos presenta despejado, tendremos condiciones ideales para observar esta hermosa lluvia de estrellas. Habrá Luna a partir de las 11h 30m y chocaremos contra uno de los filamentos más ricos dejados por el extinto cometa. Para observarlo hay que mirar después de las 10 pm hacia el Este, punto cardinal hacia donde nace el Sol. Allí observará 3 estrellas alineadas brillantes y de color azul, este es el Cinturón de Orión. Estas estrellas están metidas en un rectángulo de 3 estrellas brillantes y una de brillo medio. Proyecte una línea desde la estrella más brillante, de color azul y llamada Rigel, a la segunda estrella más brillante del rectángulo y de color rojo, Betelgause. Siga esta línea prolongándola en el cielo, hasta llegar a 2 estrellas con brillo casi idéntico, ellas son Pollux y Castor, estrellas principales de Géminis, muy cerca de ellas está el punto de donde parecen provenir los meteoros. No importa en qué lugar del cielo aparezca este, si viene de esa dirección pertenece a las Geminíadas. Como regalo, a primeras horas de la noche la Tierra contra un filamento importante del cometa Wirtanen y es muy posible que se observe una lluvia de Meteoros entre Pegasus y Piscis que estarán casi sobre nuestra cabeza luego del atardecer. Esta lluvia de meteoros se estima que produzca entre 40 y 50 meteoros por hora, la Luna no interferirá nada con la observación. Esta es la mejor de todas las lluvias de meteoros que ocurren en el año, pues son meteoros brillantes y se pueden observar toda la noche. Miembros de SOVAFA hemos descubierto varios nuevos radiantes en los últimos años. Ellos son: α Cannis Majoridas A y α Cannis Majoridas B; Colúmbidas-Lepúsidas; Vélidas; 42 Tauridas; 51 Androménidas y otros tres posibles radiantes aún por confirmar. Si usted observa esta lluvia de estrellas, cuente cuantos meteoros observa en una hora y por favor envíeme esos datos a la dirección o teléfono dado. Si desea aprender ¿cómo observar esta lluvia de estrellas?, puede buscar ¿Cómo observar radiantes meteóricos en nuestra página web: www.sovafa.com, o www.sovafa.org