Properties of starspots on CoRoT-2

Abstract

As a planet eclipses its parent star, a dark spot on the surface of the star may be occulted, causing a detectable variation in the light curve. A total of 77 consecutive transit light curves of CoRoT-2 were observed with a high temporal resolution of 32 s, corresponding to an uninterrupted period of 134 days. By analyzing small intensity variations in the transit light curves, it was possible to detect and characterize spots at fixed positions (latitude and longitude) on the surface of the star. The model used simulates planetary transits and enables the inclusion of spots on the stellar surface with different sizes, intensities (i.e. temperatures), and positions. Fitting the data by this model, it is possible to infer the spots physical characteristics. The fits were either in spot longitude and radius, with a fixed intensity, or in spots longitude and intensity, for spots of constant size. Before the modeling of the spots were performed, the planetary radius relative to the star radius was estimated by fitting the deepest transit to minimize the effect of spots. A slightly larger (3%) radius, 0.172 Rstar, resulted instead of the previously reported 0.1667 Rstar . The fitting of the transits yield spots, or spot groups, with sizes of ranging from 0.2 to 0.7 planet radius, Rp, with a mean of (0.41 +/- 0.13) Rp (~100,000 km), resulting in a stellar area covered by spots within the transit latitudes of 10-20%. The intensity varied from 0.4 to 0.9 of the disk center intensity, Ic, with a mean of (0.60 +/- 0.19) Ic, which can be converted to temperature by assuming an effective temperature of 5625 K for the stellar photosphere, the spots temperature ranges mainly from 3600 to 5000 K. The results from the spot modeling are in agreement with those found for magnetic activity analysis from out of transit data of the same star.