Understanding the effects of stellar activity on line asymmetry and RV precision

Recent advances in extreme-precision radial-velocity measurements, on the order of 30 cm/s, have made it easier to detect Earth-sized planets with longer periods around Solar-like stars. True Earth analogs, with RV semi-amplitudes of ~10 cm/s, still remain just out of reach. RV precision is now constrained primarily by stellar activity, contamination in telluric modeling, and instrumental drift. Time series spectra show changes in line asymmetry that could lead to spurious RV shifts. However, it is not clear how much stellar activity can induce a long-term and asymmetric modification to spectral line shapes. We use high signal-to-noise time-series spectra from the EXtreme PREcision Spectrometer (EXPRES) to empirically compare line shape changes of known activity-sensitive lines to more static, activity-insensitive lines. We also compare the line shape changes to symmetric line profile changes from models generated by Spectroscopy Made Easy (SME) at different effective temperatures. Lastly, we quantify the effects that these lines can have on radial velocity precision. Understanding stellar activity sensitivity for specific lines should enable us to improve RV scatter and detect Earth-analog exoplanets.