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

The discovery of the radial velocity (v -->r) signatures of planets around several solar-like stars highlights the importance of exploring the sources of v -->r variations intrinsic to the stars themselves. We study the stars in the Lick planetary survey for v -->r variations related to stellar activity: the rotation of starspots and convective inhomogeneities and their temporal evolution. We study the relationships between the weighted v -->r dispersion, ?'v (which 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 ?'v values occur among both the coolest (dMe) and the warmest (active F) stars. Values of ?'v increase with H and K emission and scale proportional to vsin${r sin}$ -->${r sin}$ -->i in G and K stars and proportional to (vsin${r sin}$ -->${r sin}$ -->i) -->1.3 in F stars. For a G star with vsin${r sin}$ -->${r sin}$ -->i ? 8-10 km s -->?1 (age ~0.3 Gyr), for example, 20 m s -->?1 ?'v 45 m s -->?1, roughly consistent with the predicted ?'v levels due to magnetic activity (Saar & Donahue). All the stars with proposed planetary companions show ?'v values typical for their spectral type, activity, and/or rotation. However, before the planetary v -->r perturbations are removed, these stars show significantly enhanced ?'v values. We develop a simple model that can predict the ?'v expected for a given star (within ? 40%) as a function of vsin${r sin}$ -->${r sin}$ -->i, spectral type, photometric variability, and macroturbulent velocity. The implications for extrasolar planet searches are discussed.