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