Late-Type Active Stars: Rotation & Companions

Rapid rotation has been established empirically as the controlling factor for the magnetic field strength and the magnetic activity level of single late-type (F-M) stars. The dynamo theories explain this fact as due to interaction between differential rotation and helical motion in the transition layer between the convective envelope and the radiative interior. The presence of a close companion even down to the size of a “hot Jupiter” could alter the physical processes responsible for the activity, by introducing longitude- and latitude-dependencies and inhomogenious chemical abundances — effects that cannot be ignored in the attempt to understand magnetic activity on late type stars. Here we present first results for the well-known, single, active star, HD27536. Binarity is established by very precise radial velocity (RV) measurements using HARPS spectra. The spectral line bisectors are examined for correlations between RV and bisector shape to distinguish between the effects of stellar activity and unseen companions.