Magnetic Activity–related Radial Velocity Variations in Cool Stars: First Results from the Lick Extrasolar Planet Survey

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Abstract
The discovery of the radial velocity () signatures of planets around several solar-like stars highlights the v r importance of exploring the sources of variations intrinsic to the stars themselves. We study the stars in the v r Lick planetary survey for variations related to stellar activity: the rotation of starspots and convective inhom-v r ogeneities and their temporal evolution. We study the relationships between the weighted dispersion, (which v j r v has first been corrected for the orbital contribution from known planets and the mean internal error), and spectral type, rotation, and activity (as measured by Ca ii H and K). We find that the largest values occur among both j v the coolest (dMe) and the warmest (active F) stars. Values of increase with H and K emission and scale j v proportional to in G and K stars and proportional to in F stars. For a G star with ≈ 1.3 v sin i (v sin i) v sin i 8-10 km s (age ∼0.3 Gyr), for example, , roughly consistent with the predicted Ϫ1 Ϫ1 Ϫ1 20 m s Շ j Շ 45 m s v levels due to magnetic activity (Saar & Donahue). All the stars with proposed planetary companions show j v values typical for their spectral type, activity, and/or rotation. However, before the planetary perturbations j v v r are removed, these stars show significantly enhanced values. We develop a simple model that can predict the j v expected for a given star (within ≈40%) as a function of , spectral type, photometric variability, and j v sin i v macroturbulent velocity. The implications for extrasolar planet searches are discussed.

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