Correcting radial velocities for long-term magnetic activity variations

We study stars in the Lick planetary survey for correlations between simultaneous measurements of high-precision radial velocities v(r) and magnetic activity (as measured in an S-IR emission index from Ca II lambda 8662). We find significant correlations in approximate to 30% of the stars. After removing linear trends between S-IR and v(r), we find that the dispersion in v(r) in these stars is decreased by an average of 17%, or approximate to 45% of the dispersion above the measurement noise. F stars and less active stars with variable Ca II H and K lines are the most successfully corrected. The magnitude of the slope of the S-IR versus v(r) relations increases proportional to v sin i and (excepting M dwarfs) tends to decrease with decreasing T-eff. We argue that the main cause of these effects is modification of the mean line bisector shape brought on by long-term, magnetic activity-induced changes in the surface brightness and convective patterns. The correlations can be used to partially correct v, data for the effects of long-term activity variations, potentially permitting study of planets around some (higher mass) younger stars and planets producing smaller stellar reflex velocities.