Stellar Spectroscopy during Exoplanet Transits:

Abstract. Differential spectroscopy during exoplanet transits permits to reconstruct spectra of small stellar surface portions that successively become hidden behind the planet. The center-to-limb behavior of stellar line shapes, asymmetries and wavelength shifts will enable detailed tests of 3-dimensional hydrodynamic models of stellar atmospheres, such that are required for any precise determination of abundances or seismic properties. Such models can now be computed for widely different stars but have been feasible to test in detail only for the Sun with its resolved surface structure. Although very high quality spectra are required, already current data permit reconstructions of line profiles in the brightest transit host stars such as HD 209458 (G0 V). 1. Context Three-dimensional and time-dependent hydrodynamic simulations are now estab-lished as realistic descriptions for the convective photospheres of various classes of stars, and must be applied in any more precise determinations of chemical abun-dances, oscillation properties, or atmospheric structure. Such models can now b