Activity-related radial velocity variation in cool stars

Planets have been detected orbiting several solar-type stars with the use of high-precision radial velocity (v(r)) measurements. While changes in v(r) can be measured with an accuracy of a few meters per second, there has been relatively little study of how other astrophysical processes, such as magnetic activity, may affect the observed velocities. In this paper, we use published data and simple models to explore the contributions to v(r) from two activity-related sources, starspots and convective inhomogeneities, as these features rotate across the disk and evolve in time. Radial velocity perturbations due to both of these sources increase with rotation and the level of surface activity. Our models indicate that for solar-age G stars, the amplitude of perturbations due to spots is A(s) less than or similar to 5 m s(-1), increasing to A(s) similar to 30-50 m s(-1) for Hyades-age G stars. If f(s) is the starspot area coverage, we find that A(s) proportional to f(s)(0.9)v sin i. The effects of convective inhomogeneities (as observed in line bisector variations) appear to depend on both rotation and spectral type. Young (active) F and G dwarfs can have convective v(r) perturbations with amplitudes A(c) less than or similar to 50 m s(-1), while v(r) amplitudes are reduced for stars with lower v sin i and cooler T-eff. We show that v(r) data from the literature display similar trends with v sin i and T-eff. A(s) and A(c) will be strongest at or near timescales related to magnetic activity variations: rotation, active region growth and decay, and activity cycles. Thus, knowledge of these timescales and typical A(s) and A(c) values are important in searching for extrasolar planets, especially those around younger, more active stars or those with small v(r) reflex amplitudes (i.e., less than or similar to 20 m s(-1)). We discuss implications of our results for current planet detections and planet search strategies.