Line-profile tomography of exoplanet transits I: The Doppler shadow of HD 189733b?

The misalignment between the orbital plane of a transiting exoplanet and the spin axis of its host star provides important insights into the system’s dynamical history. The amplitude and asymmetry of the radial-velocity distortion during a planetary transit (the Rossiter-McLaughlin effect) depend on the projected stellar rotation rate v sin I and misalignment angle λ, where the stellar rotation axis is inclined at angle I to the line of sight. The parameters derived from modelling the R-M effect have, however, been found to be prone to systematic errors arising from the time-variable asymmetry of the stellar spectral lines during transit. Here we present a direct method for isolating the component of the starlight blocked by a planet as it transits the host star, and apply it to spectra of the bright transiting planet HD 189733b. We model the global shape of the stellar cross-correlation function as the convolution of a limb-darkened rotation profile and a gaussian representing the Doppler core of the average photospheric line profile. The light blocked by the planet during the transit is a gaussian of the same intrinsic width, whose trajectory across the line profile yield