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Transcript
Deep Infrared Images of Star-Forming Cores in the Rho Oph Cloud Dale Downs Department of Physics and Astronomy University of Missouri – St. Louis Advisor: Dr. Bruce Wilking Abstract Observations were made and near-infrared photometry was performed in Cores A, E, and E1 of the Ophiuchus Cloud. Photometry of objects from these cores was plotted on infrared color-color and color-magnitude diagrams. Possible brown dwarfs were selected from the completed data and plotted along with confirmed brown dwarfs on another set of diagrams. The two goals of this research project were to identify new candidate brown dwarfs and complete missing Hubble Space Telescope infrared imagery in the Rho Oph Cloud. Background Objects in the sky emit several forms of radiation, including infrared and visible light. Visible light was the main source of astronomical research until the mid 1940’s. Astronomers could use telescopes to collect light from objects in space. Objects with temperatures between 3,000 and 10,000 K emit most of their energy in visible light. Infrared light is heat radiation. Infrared imaging allows lower temperature objects, more distant bodies, and objects obscured by dust to be seen. Infrared imaging made it possible to see into the Rho Ophiuchi Cloud. The Ophiuchus molecular cloud region is one of the nearest star formation regions to Earth. It is estimated to be 460 light years, or 140 parsecs, away. The cloud complex consists of a number of dark clouds and when measured in the CO molecule is approximately 104 M (de Geus, Bronfman, & Thaddeus 1989). At the center of cloud are several cold, dense cores with active star formation. Star formation in Rho Oph is likely triggered by the compression of the cloud by ionization fronts, supernovae explosions, and strong stellar winds from nearby B-type stars (de Geus 1992). Stellar winds can then compress the medium surrounding the young stars and produce shock-excited Herbig-Haro (HH) objects. Many of the stellar outflows may carry enough momentum and kinetic energy to induce smaller cloud cores to collapse. There are more than a 100 young stars concentrated in this area. When there is insufficient gas available to form a star in a collapsing molecular cloud, the body will not have a large enough mass or high enough temperature to sustain hydrogen fusion. What results is called a brown dwarf. They are often referred to as “failed stars.” A brown dwarf must have a mass less than 0.08 M, which is the critical mass required for hydrogen fusion. Brown dwarfs usually have a mass between 12 MJupiter and 75 MJupiter. Its core temperature must be less than 3 million degrees, again a critical value for nuclear reactions. The surface temperature is estimated to be less than 2000 K. This can cool as the brown dwarf ages. It is possible that nuclear reactions take place in the beginning of its life but they do not last long. Brown dwarfs, as a result of their low surface temperature and small size, are not very luminous. One goal of this project was to identify low-mass members in the Rho Ophiuchi Cloud that could possibly be brown dwarfs. The Hubble Space Telescope, or HST, was able to survey Ophiuchus (Allen et al. 2002), but there was a problem with NICMOS, the Near Infrared Camera and Multi-Object Spectrometer. NICMOS was said to be “the sharpest definition infrared instrument in existence”1. Solid nitrogen was used to cool NICMOS so the heat from the camera would not distort the infrared images. But the thermos flask that housed the nitrogen warped and the nitrogen evaporated. Without the nitrogen coolant, NICMOS was unable to function properly. HST was unable to observe complete infrared radiation in the 2.2 micron K-Band. Thus, the other goal of this project was to collect information for the region in the K-Band. Observations Near-infrared photometry at J (1.25 m), H (1.6 m), and Ks (2.1 m) were obtained using the University of Florida’s 20482 HgCdTe FLAMINGOS 1 camera. The observations were made at the 6.5-m MMT telescope at Mt. Hopkins, Arizona on June 15th and 16th, 2001. The scale was 0.166 pixel-1 resulting in a 5.7 x 5.7 field of view. Three fields were observed: one in the Rho Oph A molecular core and two in the Rho Oph E core. A set of observations included 5 dithers with integration times of 30 sec used for all three filters. Every third set of observations was an “off” field within the cloud that is devoid of bright objects and observed for purposes of sky-subtraction and flat-fielding. Individual frames were sky-subtracted using a median-combined image of the off field constructed from frames observed close in time. Flat-fielding was accomplished by dividing by a median-combined image of all the dark-subtracted off-field images in that filter. After adding a dc value representative of the sky background, bad pixels were masked, and the set of 5 dithers were shifted and combined into a single mosaic which covered an area of approximately 6.8 x 6.8. Positions were determined using the astrometry program ASTROM and referenced to the positions of 18 unsaturated sources in common with the survey of Barsony et al. (1997) which were referenced to HST guide stars. The 1 errors in the positions are better than 0.7 relative to the BKLT reference frame and 1.4 relative to the HST guide stars. Aperture photometry was performed on each mosaic using a 2.6 pixel aperture which is approximately 2.5 times the FWHM of the stellar profile. Zero points for the photometry were computed through comparison with the 2MASS database. 1 ESA, European Space Agency. NICMOS Returns!. http://www.spacetelescope.org/about/history/sm3b_nicmos_returns.html Results We extracted 130 sources in Core A. The completeness limits were estimated from histograms of the number of sources as a function of magnitude and were 18, 18.5, and 16.5 magnitudes at J, H, and Ks, respectively. We extracted 41 sources in Core E with completeness limits of 19, 17.7, and 16 mag. We extracted 138 sources in Core E1 with completeness limits of 19.5, 18.7, 17.5 mag. There are 7 sources in common between E and E1. There are 61 sources that appear in the Allen et al. (2002) HST paper. There are 12 sources in common with the Keck NIRC spectroscopy performed by Meyer and Wilking (2005, unpublished data). In particular is CRBR 23, which we classify as L1 and is also an x-ray source. Color-color Diagram: The color-color diagram makes a comparison of the H-K and J-H color indices. The resulting plot allows one to approximate the amount of dust in front of a star and surrounding the star (its evolutionary phase). The objects from the cores A, E, and E1 were plotted in this color-color diagram. Objects in field A are marked on the graph by the X symbol. The E1 core is represented by the ■ symbol. Field E is characterized by the ▲ symbol. Each object was labeled with consideration for its one-sigma photometric errors. The bars surrounding each object give a range that the star might fall in, with error in mind. The solid line figure at the bottom of the graph indicates the locus of points for stars on the main sequence and in the red giant phase. The upper solid line represents the red giants; and the bottom curved line represents the main sequence stars. The dashed lines, or reddening lines, carry this representation throughout the graph. If objects on this graph are reddened, they are moved above the main sequence by a field of dust in front of them. The length of the arrow line represents 10 magnitudes of visual extinction by dust. The amount of dust in front of a star can be judged by the number of lengths away from the main sequence it falls. The CTTS, Classical T Tauri Star, Locus line is displayed as the lowest of the three dashed lines; it is also angled at the bottom. Stars on this line are possible young stars with a circumstellar disk. Young stars are shifted towards this line because of emission by warm dust in their inner disks which makes them brighter in the K band. Objects to the right of the CTTS locus are brighter in the K band than expected for just a disk. They could have a disk and an envelope of dust around them. Color-Magnitude Diagram: With a color-magnitude diagram, one can either assume an age and predict the mass or assume a mass and predict the age. This color-magnitude diagram assumes an age so that we can get an estimate of the mass. Mass is marked on the reddening lines in solar masses (M). The solid lines are determined from the models of D’Antona and Mazzitelli (1997). The dashed reddening lines, with a vector of Av=10 magnitudes, were configured from the extinction law derived by Cohen et al. (1981). The completeness limit shown on the graph uses a median of the 3 fields: K=16.5 and H=18.5. The symbols on this graph represent the same fields they did in the color-color diagram. Brown Dwarf Color-Color Diagram: This graph is a color-color diagram just like the first except only confirmed brown dwarfs and possible brown dwarfs are plotted on it. The X figure represents the stars in our field that have been identified as brown dwarfs by examining their spectra. The ▲ figure plots the positions of the brown dwarf candidates. Only two of our candidates were observed through the J-Band and were able to be plotted on this graph. The rest were not bright enough to be recorded at the J-Band. Objects to the right of the middle dashed line (the lower main sequence reddening line) are in a good place to be classified as brown dwarfs but their positions could vary due to scattered light. Brown Dwarf Color-Magnitude Diagram: Brown dwarfs are displayed on this graph as well. They are represented in the same manner as the previous diagram. Brown dwarfs should fall below 0.08 solar masses on a colormagnitude graph. It is harder to determine whether objects below 0.017 solar masses are older brown dwarfs or background stars. The two X’s below this line are confirmed not to be background stars. Our candidates below this line are hard to classify. The candidate just above the 0.04 solar mass line is GY 30. It is known to be a young object with a stellar outflow and can be classified as a young brown dwarf. Summary Completing the first goal of this project, we were able to identify new brown dwarf candidates. We compared the infrared colors of these candidates with those of confirmed brown dwarfs identified through spectroscopy in an effort to predict the likelihood that the candidate is a brown dwarf. Color-color and color-magnitude diagrams were created from the infrared photometry of the region. If an object had a relatively low mass, appeared with a low luminosity on the color-color and color-magnitude diagram, and gives off x-ray radiation, it is very likely to be a newly formed brown dwarf and not a background star. The work on this project was also able to supply missing data for the Hubble Space Telescope infrared imagery of the Rho Oph Cloud. References Allen, L., Myers, P., Di Francesco, J., Mathieu, R., Chen, H., and Young, E. 2002, Astrophysical Journal, 566, 993 Barsony, M., Kenyon, S. J., Lada, E. A., & Teuben, P. J., et al. 1997, Astrophysical Journal Supplement, 112, 109 Cohen, J. G., Frogel, J. A., Persson, S. E., & Elias, J. H. 1981, Astrophysical Journal, 249, 481 D’Antona, F., & Mazzitelli, I. 1997, Mem. Soc. Aston. Italiana, 68, 807 de Geus, E., Bronfman, L., and Thaddeus, P. 1990, Astronomy & Astrophysics, 231, 137 de Geus, E. 1992, Astronomy & Astrophysics, 262, 258