Orbital period modulation in close binaries due to cyclic -dynamo activity

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
² Osservatorio Astrofisico di Catania dell'Istituto Nazionale di Astrofisica, Via Santa Sofia 78, 95125 Catania, Italy

Corresponding author: G. Rüdiger, gruediger@aip.de

Received: 10 April 2002
Accepted: 21 June 2002

Abstract

The question is answered whether dynamo-generated magnetic fields are able to produce such quadrupole terms in the gravity potential which can explain the observed cyclic orbital variation of RS CVn stars. We start with spherical dynamo models with outer convection zones but without any differential rotation, i.e. with α ²-dynamos which are known as nonoscillating. With the known anisotropic α-tensor of rapidly rotating stars the magnetic modes with the lowest dynamo numbers are nonaxisymmetric with a slow azimuthal drift. We also find, however, stable (i.e. with the lowest dynamo number) axisymmetric oscillating modes but only for a very special, highly inhomogeneous α-tensor. The dynamo model is a linear one with an arbitrary field amplitude which can be scaled in order to reproduce the observations. The star proves to be as prolate during the maximum of the toroidal field energy and it proves to be as oblate during the maximum of the poloidal field energy. In the time average the influence of the toroidal field dominates and the star is slightly prolate. From the computed temporal variations of the gravitation quadrupole moment a magnetic field of more than 10⁵ G is found in order to produce the period modulation of order 10^-5 which has been observed.