Prof Suzanne Aigrain

Abstract:

Aims. The Antarctica Search for Transiting ExoPlanets (ASTEP) program was originally aimed at probing the quality of the Dome C, Antarctica for the discovery and characterization of exoplanets by photometry. In the first year of operation of the 40 cm ASTEP 400 telescope (austral winter 2010), we targeted the known transiting planet WASP-19b in order to try to detect its secondary transits in the visible. This is made possible by the excellent sub-millimagnitude precision of the binned data. Methods. The WASP-19 system was observed during 24 nights in May 2010. Once brought back from Antarctica, the data were processed using various methods, and the best results were with an implementation of the optimal image subtraction (OIS) algorithm. Results. The photometric variability level due to starspots is about 1.8% (peak-to-peak), in line with the SuperWASP data from 2007 (1.4%) and higher than in 2008 (0.07%). We find a rotation period of WASP-19 of 10.7 ± 0.5 days, in agreement with the SuperWASP determination of 10.5 ± 0.2 days. Theoretical models show that this can only be explained if tidal dissipation in the star is weak, i.e. the tidal dissipation factor Q'ẫ... > 3×107. Separately, we find evidence of a secondary eclipse of depth 390 ± 190 ppm with a 2.0σ significance, a phase that is consistent with a circular orbit and a 3% false positive probability. Given the wavelength range of the observations (420 to 950 nm), the secondary transit depth translates into a day-side brightness temperature of 2690 -220+150 K, in line with measurements in the z′ and K bands. The day-side emission observed in the visible could be due either to thermal emission of an extremely hot day side with very little redistribution of heat to the night side or to direct reflection of stellar light with a maximum geometrical albedo Ag = 0.27 ± 0.13. We also report a low-frequency oscillation in phase at the planet orbital period, but with a lower limit amplitude that could not be attributed to the planet phase alone and that was possibly contaminated with residual lightcurve trends. Conclusions. This first evidence of a secondary eclipse in the visible from the ground demonstrates the high potential of Dome C, Antarctica, for continuous photometric observations of stars with exoplanets. These continuous observations are required to understand star-planet interactions and the dynamical properties of exoplanetary atmospheres. © 2013 ESO.

Abstract:

Context. The CoRoT space mission has been searching for transiting planets since the end of December 2006. It has led to the detection of about twenty Jupiter-size planets and three planets with radius Rp≲5 R⊕ The latter are CoRoT-7b, the first super-Earth observed in transit, and two validated Neptunes, CoRoT-24b and c, in a multiple system. Aims.We aim to investigate the capability of CoRoT to detect small-size transiting planets in short-period orbits, and to compare the number of CoRoT planets with 2.0 = Rp = 4.0 R. with the occurrence rate of small-size planets provided by the distribution of Kepler planetary candidates. Methods.We performed a test that simulates transits of super-Earths and Neptunes in real CoRoT light curves of six long observational runs and searches for them blindly by using the transit detection pipeline developed at the Laboratoire d'Astrophysique de Marseille. Results.The CoRoT detection rate of planets with radius between 2 and 4 R. and orbital period P = 20 days is 59% (31%) around stars brighter than r′ = 14.0 (15.5). The vast majority of the missed planets went undetected because of a low transit signal-to-noise ratio (S/N). However, in some cases, additional instrumental or astrophysical noise may prevent even transits with relatively high S/N, i.e. S/N = 10, from being revealed. By properly taking the CoRoT detection rate for Neptune-size planets (2 = Rp = 4 R.) and the transit probability into account, we found that according to the Kepler planet occurrence rate, CoRoT should have discovered 12 ± 2 Neptunes orbiting G and K dwarfs with P = 17 days in six observational runs. This estimate must be compared with the validated Neptune CoRoT-24b and five CoRoT planetary candidates in the considered range of planetary radii, the nature of which is still unsolved. We thus found a disagreement with expectations from Kepler at 3s or 5s, assuming a blend fraction of 0% (six Neptunes) and 100% (one Neptune) for these candidates. Conclusions.This underabundance of CoRoT Neptunes with respect to Kepler may be due to a) an underestimate of the uncertainty on the Kepler planet occurrence; b) an underestimate of the false-positive probability of the Kepler small-size planetary candidates; c) an overestimate of our prediction of CoRoT Neptunes related to the number of G and K dwarfs observed by CoRoT; or d) different stellar populations probed by the two space missions. Regardless of the origin of the disagreement, which needs to be investigated in more detail, the noticeable deficiency of CoRoT Neptunes at short orbital periods seems to indirectly support the general trend found in Kepler data, i.e. that the frequency of small-size planets increases with increasing orbital periods and decreasing planet radii. © 2012 ESO.