Spot signatures of a solar-type dynamo in close binary stars

Abstract

Magnetic activity signatures in the atmosphere of active stars can be used to place constrains on the underlying processes of flux transport and dynamo operation in its convective envelope. The 'solar paradigm' for magnetic activity suggests that the magnetic field is amplified and stored at the base of the convection zone. Once a critical field strength is exceeded, perturbations initiate the onset of instabilities and the growth of magnetic flux loops, which rise through the convection zone, emerge at the stellar surface, and eventually lead to the formation of starspots and active regions. In close binaries, the proximity of the companion star breaks the rotational symmetry. Although the magnitude of tidal distortions is rather small, non-linear MHD simulations have nevertheless shown in the case of main-sequence binary components that they can cause non-uniform surface distributions of flux tube eruptions. The present work extends the investigation to post-main-sequence components to explore the specific influence of the stellar structure on the surface pattern of erupting flux tubes. In contrast to the case of main-sequence components, where the consistency between simulation results and observations supports the presumption of a solar-like dynamo mechanism, the numerical results here do not recover the starspot properties frequently observed on evolved binary components. This aspect points out an insufficiency of the applied flux tube model and leads to the conclusion that additional flux transport and possibly amplification mechanisms have to be taken into account.