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

¹ Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: dreizler@astro.physik.uni-goettingen.de
² Gymnasium am Bötschenberg, Am Bötschenberg 11, 38350 Helmstedt, Germany

Received: 23 July 2008
Accepted: 23 February 2009

Abstract

Context. The detection of extrasolar planets' atmospheres requires very demanding observations. For planets that cannot be spatially separated from their host stars, i.e. the vast majority of planets, the transiting planets are the only ones to allow their atmospheres to be probed. This is possible from transmission spectroscopy or from measurements taken during the secondary eclipse. An alternative is to measure 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 toward also probing planetary atmospheres 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 detailed phase-resolved synthetic spectra modeling the partly eclipsed stellar surface during the transit. With these spectra we can obtain Rossiter-McLaughlin curves for different wavelength regions and for a wavelength-dependent planetary radius. Curves from regions with high and low contributions of absorption lines within the planetary atmosphere can be compared. Observable quantities are derived from these differential effects.

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 s^-1 occur. The wavelength dependency of the planetary atmospheres over the NaD-doublet produces a differential effect in the Rossiter-McLaughlin curve of 1.5 m s^-1 for a star with a rotation velocity of 4.5 km s^-1, which increases to 4 m s^-1 for twice the rotation velocity.

Conclusions. As a tool for probing planetary atmospheres the Rossiter-McLaughlin effect requires phase-resolved, high signal-to-noise, high-resolution spectra taken with a stabilized spectrograph in order to obtain reliable results for slowly rotating (<10 m s^-1) planet host stars. Stars with spectral type earlier than about F5 are a bit less demanding since the typically higher rotation velocity increases the amplitude of the effect to about 15 m s^-1 for a star with v sin i = 25 km s^-1.