Stellar surface magnetoconvection as a source of astrophysical noise. III. Sun-as-a-star simulations and optimal noise diagnostics

Stellar surface magnetoconvection (granulation) creates asymmetries in the observed stellar absorption lines that can subsequently manifest themselves as spurious radial velocities (RVs) shifts. In turn, this can then mask the Doppler reflex motion induced by orbiting planets on their host stars and represents a particular challenge for determining the masses of low-mass, long-period planets. Herein, we study this impact by creating Sun-as-a-star observations that encapsulate the granulation variability expected from 3D magnetohydrodynamic simulations. These Sun-as-a-star model observations are in good agreement with empirical observations of the Sun but may underestimate the total variability relative to the quiet Sun due to the increased magnetic field strength in our models. We find numerous line profile characteristics that linearly correlate with the disk-integrated convection-induced velocities. Removing the various correlations with the line bisector, equivalent width, and the V asy indicator may reduce ~50%–60% of the granulation noise in the measured velocities. We also find that simultaneous photometry may be a key diagnostic, as our proxy for photometric brightness also allowed us to remove ~50% of the granulation-induced RV noise. These correlations and granulation-noise mitigations break down in the presence of low instrumental resolution and/or increased stellar rotation, as both act to smooth the observed line profile asymmetries.