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, sigma(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 sigma(v)' values occur among both the coolest (dMe) and the warmest (active F) stars. Values of sigma(v)' increase with H and K emission and scale proportional to v sin i in G and K stars and proportional to (v sin i)(1.3) in F stars. For a G star with zi sin i approximate to 8-10 km s(-1) (age similar to 0.3 Gyr), for example, 20 m s(-1) less than or similar to sigma(v)' less than or similar to 45 m s(-1), roughly consistent with the predicted sigma(v)' levels due to magnetic activity (Saar & Donahue). All the stars with proposed planetary companions show sigma(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 sigma(v)' values. We develop a simple model that can predict the sigma(v)' expected for a given star (within approximate to 40%) as a function of v sin i, spectral type, photometric variability, and macroturbulent velocity. The implications for extrasolar planet searches are discussed.