The Sun’s center-to-limb variation and its impact on radial velocity and transit observations

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Accurate knowledge about stellar surface spectra is required for the search for extrasolar planets with the radial velocity method, the investigation of exoplanet atmospheres, and for understanding surface convection in stars and the Sun. The variability of spectral line profiles across the surface and in time must be understood in great detail in order to find and correctly interpret the subtle signatures of extrasolar planets. Empirical information about line profiles from spatially resolved observations can only be obtained for the Sun from direct observations, but only limited data at the required accuracy are available and only for a small number of spectral lines. We propose to obtain reference observations of the Sun at different magnetic field strengths for the full optical and near-infrared wavelength range (450-2300\,nm) across the solar surface at different limb angles. We will perform a detailed analysis of convective blueshift for different limb angles and magnetic fields, and the data will provide crucial information to make progress in the methodologies used for the search for exoplanets and the analysis of exoplanet atmospheres. The data shall be made available in the form of electronic atlases to the scientific community. We will provide a tool for the simulation of line profiles and RVs in stars with parameterized surface properties and for transit simulations, and we will apply the tool to archive radial velocity data from stars to search for planetary signatures hidden by intrinsic stellar variability.