Predicting convective blueshift and radial-velocity dispersion due to granulation for FGK stars
Résumé
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). Conv ectiv e blueshift, which results from small-scale convection at the surface of Sun-like stars, is rele v ant for Earth-twin detections as it exhibits Doppler noise of the order of 1 m s -1 . Here, we present a simple model for conv ectiv e blueshift based on fundamental equations of stellar structure. Our model successfully matches observations of conv ectiv e 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 surv e ys and radial velocity follow-up programmes for TESS, CHEOPS, and the upcoming PLATO mission.
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