Magnetic fields of red giants and supergiants: a review of spectropolarimetric observations

Abstract

Today, magnetic fields have been reliably detected in many classes of stars with convective envelopes, from young T~Tauri stars to supergiants. We present an overview of the results obtained with high-precision spectropolarimetric observations of selected single F0-M0 giants and supergiants. The measurements of the magnetic field in these objects were started in 1989 at the 2.6-meter ZTSh telescope of the Crimean Astrophysical Observatory. To date, weak magnetic fields have been recorded in nearly four dozen of slowly rotating red giants. The longitudinal component of the field in several cases reaches a few tens of gauss. A spectropolarimetric survey of red supergiants includes three dozen objects. The magnetic field was detected in dozen of them. For one of these targets, $\epsilon$~Gem, the magnetic field up to 10~G was reported. Because the magnetic field is frozen into the plasma, it is expected that the magnetic field of giants and supergiants should not exceed one gauss because stars have increased in size after the main sequence. Therefore, the main conclusion from the results of spectropolarimetric surveys of giants and supergiants with convective envelopes is that the most probable mechanism for the generation and amplification of the magnetic field in these objects is the dynamo action.