AM Her Stars: The Maintenance of Synchronism

Abstract

The problem of the maintenance of synchronism is addressed. Since the inductive synchronizing torque vanishes at corotation, there must be another torque present to balance the accretion torque. In general, a 3D torque is required and the balance must be stable. Because the secondary star has at least a convective envelope, and is rapidly rotating due to tidal synchronization, it is likely to have a large-scale magnetic field generated by dynamo action. The interaction of such a field with the magnetic primary can produce a torque which can balance the accretion torque, with stable orientations. Certain restrictions can result related to the surface polar strength and the orientation of the secondary’s magnetic field.

Another possible balancing torque results if the primary is distorted from spherical symmetry due to non-radial internal magnetic forces, caused by the electric current sources of its magnetic field. A tidal torque acts on the distorted star and this can balance the magnetic torque to produce a synchronous state, if these torques significantly exceed the accretion torque. This can occur if the secondary has a surface magnetic field of a few kG. The effects of these torques in relation to the orbital evolution are also considered.