Rotation and magnetic activity of the Hertzsprung-gap giant 31 Comae

Abstract

Context. The single rapidly-rotating G0 giant 31 Comae has been a puzzle because of the absence of photometric variability despite its strong chromospheric and coronal emissions. As a Hertzsprung-gap giant, it is expected to be at the stage of rearranging its moment of inertia, hence likely also its dynamo action, which could possibly be linked with its missing photospheric activity. Aims. Our aim is to detect photospheric activity, obtain the rotation period, and use it for a first Doppler image of the star’s surface. Its morphology could be related to the evolutionary status. Methods. We carried out high-precision, white-light photometry with the MOST satellite, ground-based Stromgren photometry with automated telescopes, and high-resolution optical echelle spectroscopy with the new STELLA robotic facility. Results. The MOST data reveal, for the first time, light variations with a full amplitude of 5 mmag and an average photometric period of 6.80 ± 0.06 days. Radial-velocity variations with a full amplitude of 270 m s −1 and a period of 6.76 ± 0.02 days were detected from our STELLA spectra, which we also interpret as due to stellar rotation. The two-year constancy of the average radial velocity of +0.10 ± 0.33 km s −1 confirms the star’s single status, as well as the membership in the cluster Melotte 111. A spectrum synthesis gives Teff = 5660 ± 42 K, log g = 3.51 ± 0.09, and [Fe/H] = −0.15 ± 0.03, which together with the revised Hipparcos distance, suggests a )