Constructing A Physically Motivated Model of Activity-Driven Solar and Stellar Radial Velocities

Analysing the periodic radial-velocity variations of a star caused by an orbiting planet is a highly successful way of inferring the masses of exoplanets around bright, nearby stars. A major limitation to this method comes from rotationally modulated stellar activity signals that hide orbits of small exoplanets by creating variations in radial-velocity measurements. We look to the Sun as a test of concept to understand the effects of solar and stellar activity on radial-velocity variations. We construct a physically grounded model for the suppression of convective blueshift and rotation of active regions across the solar/stellar disk. Additionally and for the first time, we model horizontal velocity flows in and around active regions (Evershed and moat flows directed radially outward from sun/starspots, inflows around plage regions). We use SORCE photometry and HARPS-N Ca II H&K line emission as proxies for these physical effects, and fit to measured radial-velocity variations of the Sun seen as a star from the HARPS-N spectrograph. We also model radial-velocity measurements of the rocky-planet host star CoRoT-7, using CoRoT photometry and HARPS Ca II H&K emission observations. We apply our model for stellar activity and compare to previous models that did not account for horizontal velocity flows. This work is an essential step towards modelling the physical effects of stellar activity on radial-velocity variations, which is crucial to uncovering Earth-like exoplanets orbiting Sun-like stars.

This work is supported under the NSF-REU solar physics program at SAO, grant number AGS-1560313, performed in part under contract with the California Institute of Technology (Caltech)/Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute (R.D.H.), and supported in part by NASA award number NNX16AD42G, the Smithsonian Institution, NASA Heliophysics LWS grant NNX16AB79G (S.H.S.) and the HARPS-N project.