On the possibility of detecting extrasolar planets’ atmospheres with the Rossiter-McLaughlin-effect

Context. The detection of extrasolar planets’ atmospheres requires very demanding observations. For planets that can not be spatially separated from their host stars, i.e. the vast majority of planets, the transiting planets are the only ones allowing to probe their atmospheres. This is possible from transmission spectroscopy or from measurements taken during secondary eclipse. An alternative is the measurement of the Rossiter-McLaughlin-effect, which is sensitive to the size of the planetary radius. Since the radius is wavelength-dependent due to contributions of strong planetary absorption lines, this opens a path to probe planetary atmospheres also with ground-based high-resolution spectroscopy. Aims. The major goal of our numerical simulations is to provide a reliable estimate of the amplitude of the wavelength-dependent Rossiter-McLaughlin-effect. Methods. Our numerical simulations provide phase resolved synthetic spectra modeling the partly eclipsed stellar surface during the transit in detail. Using these spectra we can obtain Rossiter-McLaughlin-curves for different wavelength regions and for a wavelengthdependent planetary radius. Curves from regions with high and low contributions of absorption lines within the planetary atmosphere can be compared. From these differential effects observable quantities are derived. Results. We applied our simulations to HD 209458. Our numerical simulations show that a detailed treatment of the limb darkening for the synthetic spectra is important for a precise analysis. Compared to a parameterized limb darkening law, systematic errors of 6 m sec−1 occur. The wavelength dependency of the planetary atmospheres over the NaD-doublet produce a differential effect in the Rossiter-McLaughlin-curve of 1.5 m sec−1 for a star with a rotation velocity of 4.5 km sec−1 which increases to 4 m sec−1 for twice the rotation velocity. Conclusions. The Rossiter-McLaughlin-effect as tool to probe planetary atmospheres requires phase resolved, high signal-to-nose, high-resolution spectra taken with a stabilized spectrograph in order to obtain reliable results for slowly rotating (< 10 m sec−1) planet host stars. Stars with spectral type earlier than about F5 are a bit less demanding since the typically higher rotation velocity increase the amplitude of the effect to about 15 m sec−1 for a star with v sin i = 25 km sec−1.