Predicting convective blueshift and radial-velocity dispersion due to granulation for FGK stars

Abstract

ABSTRACT To detect Earth-mass planets using the Doppler method, a major obstacle is to differentiate the planetary signal from intrinsic stellar variability (e.g. pulsations, granulation, spots, and plages). Convective blueshift, which results from small-scale convection at the surface of Sun-like stars, is relevant for Earth-twin detections as it exhibits Doppler noise of the order of 1 $\rm m\, s^{-1}$. Here, we present a simple model for convective blueshift based on fundamental equations of stellar structure. Our model successfully matches observations of convective blueshift for FGK stars. Based on our model, we also compute the intrinsic noise floor for stellar granulation in the radial-velocity observations. We find that for a given mass range, stars with higher metallicities display lower radial-velocity dispersion due to granulation, in agreement with magnetohydrodynamic simulations. We also provide a set of formulae to predict the amplitude of radial-velocity dispersion due to granulation as a function of stellar parameters. Our work is vital in identifying the most amenable stellar targets for Extreme Precision Radial Velocity surveys and radial velocity follow-up programmes for TESS, CHEOPS, and the upcoming PLATO mission.