Modeling granulation's effect on stellar line shapes and EPRV surveys with GRASS: The GRanulation and Spectrum Simulator

Granulation in the atmospheres of cool stars introduce changes to the shapes of absorption lines distinct from pure Doppler shifts. For stellar astronomers, these line-shape changes (quantified as line bisectors) are useful for studying convective motions in stellar atmospheres.  For astronomers who study exoplanets, these changes pose a significant obstacle to the longstanding goal of detecting and characterizing Earth-like exoplanets, even despite advances in radial velocity spectrographs. To further understand how granulation in the atmospheres of Sun-like stars contributes to variability of stellar spectra, we have developed a package (GRASS, the GRanulation And Spectrum Simulator; https://github.com/palumbom/GRASS) which uses very high spectral resolution, spatially-resolved observations of the Sun to empirically model granulation-induced changes in the shapes of stellar lines. We present this program, and evaluate our granulation model against observations of the disk-integrated Sun. We explore how granulation differentially affects lines with differing depth, excitation potential, and magnetic sensitivity. Additionally, we use these results to inform  which strategies for mitigating stellar variability are sensitive to granulation-driven variability.