Download LT telescope proposal (2011a)

186-LT11/11A
INSTITUTO DE ASTROFÍSICA DE CANARIAS
38200 La Laguna, Tenerife, (Islas Canarias)
Teléfono: 922 605 200 - Fax: 922 605 210
PETICIÓN DE TIEMPO DE OBSERVACIÓN (CAT NOCTURNO)
Observatorios del Roque de Los Muchachos y del Teide
1. Título (máximo diez palabras)
Characterizing stellar activity in transit observations
2. Datos personales
2.1. Investigador principal
Hannu Parviainen
Instituto de Astrofísica de Canarias
2.2. Coinvestigadores
Markus Rabus
Universidad de Chile
Hans J. Deeg
Instituto de Astrofísica de Canarias
Roi Alonso Sobrino
Observatoire de Genève
2.3. Persona de contacto
Nombre: Hannu Parviainen
Centro:
Instituto de Astrofísica de Canarias
Teléfono: +34618772810
Fax:
Dirección:
Email:
ESPAÑA
[email protected]
3. Resumen del programa (máximo 100 palabras)
We request time for observations to characterize the effects of stellar activity onto planetary transits. Our target is the planet-host star Corot-2, selected
due to its strong stellar activity, good observability and for its planet having parameters that are difficult to explain with current models. Observations
should also lead to strategies to better correct for effects from activity in the interpretation of transit light curves and to improve future observational
procedures of transit events.
4. ¿Es Tiempo de Colaboración con GTC?
Sí
No
X
5. Descripción de la Colaboración con GTC
6. Campo de investigación
Estrellas de Baja Masa Objetos Subestelares y Sistemas Planetarios
7. ¿Es continuación de programa que obtuvo tiempo?
Sí
X
No
8. ¿Es este programa parte de un trabajo de tesis?
Sí
X
No
Título
Characterization of
Exoplanetary Systems with
Transiting Planets
Doctorando
Hannu Parviainen
9. ¿Necesita tiempo en semestres siguientes?
Sí
10. Tiempo de Observación solicitado
Telescopio Instr. Noches Horas Luna Fechas óptimas
LT
RATCam
27
G See Table 8.1
Cfg. instr.: Filters Bessel V, z, Halpha
I. 2ario:
10.1. Justificación fechas imposibles
Time-critical observations of planet transits
Supervisor
Hans Deeg, Juan Antonio
Belmonte Aviles
No
X
Fechas imposibles
All others
I. visit.:
11. Comentarios
Repetition of previously approved program which was seriously affected by instrument failures.
Main observations should be at three dates selected from table in 8.1; plus 20 minutes of observations every
night for a week before and after the selected dates.
12. Fecha/Firma Investigador Principal
Fecha: 01-10-10
Fecha/Firma Director Centro
Fecha: 01-10-10
7. Justificación científica
Since the advent of precise ground and space-based transit observations, occasional deviations from
the ideal shape of a planetary transit have been explained as the possible imprints from ‘star spots’ on
the central star; when the planet occults stellar areas with surface brightnesses that are not described
by limb-darkening relations. However, only with the advent of long-coverage light curves of high precision from the CoRoT mission, the characterization of the stellar activity has been recognized to be of
major influence in the precise interpretation of the detected planetary transits. Of the six CoRoT-planets published to date, three show significant stellar activity (Fig. 2), mainly caused by the existence of
large spotted areas that change on time-scales of weeks and rotate with the star in an out of the view.
The most outstanding example and best investigated case to date is CoRoT-2, an active star by all
standards whose light curve is dominated by activity that is strongly modulated by the stellar rotation
period (Alonso et al., 2008). The host-star of the first detected Super-Earth planet, CoRoT-7b (Leger
et al., 2009), also shows strong stellar activity that interferes and complicates the interpretation of its
transit lightcurve. At the same time that the interplay of activity and transits is recognized, CoRoT-like
long duration light curves are opening new lines of investigation into activity using time-domain data
(e.g. Mosser et al., 2009)
The most direct consequence of activity onto the derived planet parameters is a significant underestimation in the planetary size, as has been shown by Czesla et al. (2009) in the case of CoRoT-2b.
This underestimation comes from two sources: averaging over the entire transit without removing the
intensity bumps due to the stellar activity would make the transit depth (∆F) shallower; the derived
planet-radius smaller by ∆F1/2 and increase the planet density by ∆F 3/2. Especially the influence of
activity on planetary density estimates, estimated by Czesla et al. to be of 10% for CoRoT-2b, is of
relevant magnitude for planetary models. A further source for deviant radius estimates may be due to
deviations in the stellar disk brightness; recent works (Huber et al. 2010, Silva-Valio et al. 2009 combined with Lanza et al. 2009) indicate that the strip occulted by the CoRoT-2’s planet has a lower surface brightness than the rest of the star; similar to the well-known tendency of solar spots to occur
preferentially at mid-latitudes. However, this interpretation implies that the planet would still be larger
than thought; its large size (1.47RJup) already being difficult to explain with planetary models. Alternatively, star-planet interaction may play an important role for a system with massive planet orbiting
very close to the star (Pagano et al. 2009). Deviations from uniform stellar brightness might arise from
hot spots or from stellar strip brighter than the average stellar flux, which could have been induced by
the planet. In this case, the planet would really be smaller, more in agreement with models.
A second consequence of activity signatures are the errors they induce into measurements of the
transit duration and the transit timing. Transit duration is related to the derived impact parameter and
its variation progresses to practically all physical parameters of a planet system. Transit mid-times
have been used by numerous authors (e.g. Winn et al., 2007, Holman et al., 2009, Rabus et al., 2009b)
to analyse for variations in periodicity, and activity may modify transit ingress and egress flanks,
thereby contaminating precise measurements of transit times.
The objective of our proposal is the creation of a set of tools and procedures that allow for a
better understanding and improved correction against effects from stellar activity on transit
lightcurves. Such tools should become valuable for the analysis of small-sized planets, where individual activity related features cannot be recognized; but they should also enable the correction of results from larger planets and give guidelines for the planning of future observing campaigns. We
choose CoRoT-2b as an example for its high level of activity, for being a very well described system
(but still riddled with a planet radius poorly explained by models), and for being a relatively bright
(V=12.7) target with a deep (~3%) transit.
We propose to observe three CoRoT-2b transits in multiple filters (Bessel V, z’ and Hα) simultaneously, which should give answers to the following questions:
• Which colors are least affected by activity or deliver the most reliable values for planet models?
• Could combinations of colors be used as indicators for stellar activity?
• Could any combinations of colors be used to correct for stellar activity?
Among the filters chosen, Hα should serve as the tracer of the activity-level on short time-scales during the transit. Between Hα and either of the broad-band filters, one might expect an anticorrelated behaviour if the planet passes over active regions, as these are brighter than the average stellar Hα
emission; if the planet passes over hot spots instead, a correlated behaviour would be expected. The
two continuum filters have the following further functions: Bessel V and z’ span a relatively large
wavelength range; a combination of them may allow an approximate reconstruction of the lightcurve
as ‘seen’ by CoRoT; the wide span will amplify differences between these colors both form activity
features as well as from the significantly different limb-darkening of the star; and z’ on its own should
provide a light-curve with a diminished signal of the activity signatures.
For the interpretation of the transit data, it is important to determine the level of stellar activity
during the transit. To this end, we request short nightly observations of CoRoT-2a for a week before
and after the scheduled transit observation (This is the major reason that the Liverpool telescopes has
been requested; such observations are difficult to obtain elsewhere). This extended observation span
would allow a recreation of a short section of the activity-modulated curve as shown in Fig. 2; activity
levels and estimations for spot-sizes from such a lightcurve will then be extracted following the procedures outlined in Mosser et al. (2009).
This is a repetition of a program approved for semester 2010A which was seriously affected
by instrument failures. We obtained only one full transit out of three approved transit observations, shown in Fig. 1, and 37 nights of short calibration observations. The short nights allow us
to improve the characterization of the CoRoT-2 activity, but a single transit is not enough for
detailed analysis of the effects due to the activity on the transit depth. Nevertheless, as the instrument problems have now been fixed, the high quality of the single obtained transit lightcurve encourages us to redo the program
References
Alonso et al., 2008, A&A, 482, L21
Holman et al., 2007, ApJ, 664, 1185
Huber et al., 2010, A&A, 514, 39H
Lanza et al., 2009, A&A, 493, 193L
Mosser et al., 2009, A&A, 506, 245
Pagano et al.,2009, arXiv:0905.3633v1
Rabus et al., 2009a, A&A, ,494, 391
Rabus et al., 2009b, A&A, 508, 1011
Silva-Valio et al., 2010, A&A, 510, A25
Winn et al., 2007, ApJ, 657, 1098
Gráficas
Figure 1: Results from the 2010A observation run for three filters and color differences. Unfortunately, the
observations were affected by instrument failures of the Liverpool Telescope, and we obtained only one transit.
Together with the transit is shown the mean stellar brightness (with 3σ errorbars) for the short reference nights
before and after the transit.
Figure 2: From top to bottom, lightcurves of Corot-2 and 7 (Alonso et al. 2008, Legér et al. 2009). In both
cases, percent-level brightness variations due to stellar activity are visible and modulated by the stars’ rotation
periods - very apparent in Corot-2, with Prot=4.5-5 days; less apparent in Corot-7 (Prot= 23d, inferred from
spot-induced dips, as pointed out by arrows). The planetary transits are only visible in Corot-2, whereas they
are too weak (∆F/F ~0.035%) to be recognized in Corot-7.
7.2. ¿Tiene el grupo experiencia observacional previa con instrumentación similar?
•
Yes. This is a repetition of a program approved previously for semester 2010A affected by
instrument failures. The instrumental problems have now been fixed, and we have gained a
good wealth of experience with the Liverpool Telescope to optimize our observations further.
7.3. Enumerar algún artículo del equipo investigador relevante a esta propuesta (no dar más de
seis referencias)
8. Descripción del plan de obtención, reducción y análisis de los datos
8.1. Justificación de la cantidad de tiempo solicitado
Nombre
A.R. (hh:mm:ss)
Dec
V
Tipo Esp.
CoRoT-2
19:27:06.5
+01:02:01.5
12.57
G7V
We want to observe lightcurves of three full transits of CoRoT-2, and perform short (20min) calibration observations of CoRoT-2 every night during the week preceding and following a transit.
Three transits are requested in order to improve the reliability of any conclusions derived from the
data, considering that the level of stellar activity that might be very different at each transit. Times for
transits in 2011A with prioritization are given in Table 1. Each transit lasts 2.33 hours and in nights
with transits, the target should be observed an additional hour before and after the transit (twilight and
altitude limits permitting); that is, 4.33 hours are needed in each night with a transit.
The calibration observations in nights preceding/following those with transits will take a total of
14 x 0.33h = 4.66 hours per transit. So, observations of one transit require a total of 9 hours. For 3
transits this leads to a maximum of 27 hours required. If nights with transits are scheduled less
then two weeks apart, there are of course fewer nightly calibration observations needed.
For example, allocating the first three transits from the table below, the total time would be 21.66
h. The exposures will cycle through V, z’ and Hα filters, with exposure times of 12, 12 and 60sec, respectively, which should deliver a S/N of 500 in good-average conditions. 2x2 binning should be used
to shorten read-out times.
20110513
04:16
1
20110527
02:54
2
20110603
02:13
1
20110610
01:32
2
20110624
00:11
2
20110630
23:30
1
20110707
22:50
2
20110727
02:59
1
Table 1: Transit midtimes in UT time (yyyymmdd hh:mm priority). Three nights should be picked from the table,
taking into account the priority (1 or 2). Observations should start approx. 2h10min before and end 2h10m after
the time indicated.
8.3. Una vez comenzado el programa, ¿necesitará tiempo adicional en semestres sucesivos?
•
No
8.4. ¿Es la presente propuesta continuación de un programa (igual o relacionado) que ya
obtuvo tiempo anteriormente?
•
Yes, the previous program in 2010A failed becouse of instrument failures.
8.5. Programa alternativo: titulo y breve exposición.
•
We do not require extremely good conditions for the observations.
8.6. Otras peticiones de observación indicando necesidad de realización simultánea:
•
No
9. Anteriores tiempos de observación concedidos por el CAT al
investigador principal durante los tres últimos años.
Telescopio
Fecha
Comentarios
LT
2/2010-7/2010
The original program in
2010A.
IAC80
~70h in 2010 (A&B)
CoRoT follow-up led
by B. Tingley
TCS
28 nights in 2010B
NOT
5 nights in 2010B
N Publicación
1
Publicaciones:
1. Bouchy, F., et al. (including Parviainen, Deeg and Alonso), 2011, “Transiting exoplanets from
the CoRoT space mission. XV. CoRoT-15b: a brown dwarf transiting companion”, A&A, accepted
10. Cuestiones críticas en relación a este programa
Main observations will last ~4.3 hours each at the three dates selected from Table 1; plus short calibration observations every night for a week before and after the selected dates.