Nonaxisymmetric cool spot distributions and dynamo action in close binaries

¹ Department of Mathematics, Manchester University, Manchester M13 9PL, UK
² Institute of Astronomy and Space Physics, Uppsala University, 75120, Uppsala, Sweden e-mail: piskunov@astro.uu.se
³ Department of Physics, Moscow State University, 119899, Moscow, Russia e-mail: sokoloff@dds.srcc.msu.su

Corresponding author: D. Moss, moss@ma.man.ac.uk

Received: 8 July 2002
Accepted: 19 September 2002

Abstract

We investigate the nonaxisymmetric magnetic field distribution in the components of close binary systems, generated by a stellar dynamo with a nonaxisymmetric distribution of the α-coefficient, proportional to the mean helicity of the convective motions in the stellar convective envelope. The nonaxisymmetry of α is assumed to be connected with interaction with the binary companion, i.e. the reflection effect and tidal interaction. The degree of asymmetry is estimated from the known parameters of ER Vulpeculae, an active RS CVn-type star. We demonstrate that the synchronization of rotation and orbital motion of ER Vulpeculae due to tidal interaction is sufficient to allow nonaxisymmetric magnetic structures to survive in the presence of smoothing by differential rotation. The dynamo driven nonaxisymmetric magnetic structures are usually steady in the reference frame connected with the α-spot. However, we found a resonant excitation of an oscillatory, strongly nonaxisymmetric, configuration, driven by the joint action of the α-effect and differential rotation. The magnetic structures obtained are predominantly manifested on the stellar surface in the form of spots of radial magnetic field. Their form and size are similar to the form and size of cool spots observed on the surfaces of both components of ER Vulpeculae, and their displacements from the α-spots are comparable to that of the observed cool spots from the hot spots connected with the reflection effect. Possible relevant observations are discussed.