The secondary eclipses of WASP-19b as seen by the ASTEP 400 telescope from Antarctica^⋆,⋆⋆

¹ Université de Nice Sophia-Antipolis, Observatoire de la Côte d’Azur, CNRS UMR 7293, 06108 Nice Cedex 2, France
e-mail: Lyu.Abe@unice.fr
² School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
³ Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁴ Space Telescope Science Institute, Baltimore, MD 21218, USA
⁵ DLR Institute for Planetary Research, 12489 Berlin, Germany
⁶ Harvard – Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁷ Optique et Vision, 6 bis avenue de l’Estérel, BP 69, 06162 Juan-Les-Pins, France
⁸ Observatoire de Haute-Provence, Université d’Aix-Marseille & CNRS, 04870 Saint Michel l’Observatoire, France
⁹ Department of Physics, University of Oxford, Oxford OX1 3RH, UK

Received: 7 September 2012
Accepted: 1 March 2013

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'_★ > 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 A_g = 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.