Simulations of Angular Momentum Evolution in Wind-Fed CV Precursors

Abstract

It is thought that the secondary stars in cataclysmic variables (CVs) may undergo a period of mass loss in the form of a wind during the evolution of the system (Mullan et al. 1992). This wind is thought to magnetically brake the secondary star with a time-scale ~ 108 yr (e.g. van Paradijs 1986). When the secondary’s spin has been brought close to synchronism with the orbit it is possible for tidal torques to lock the secondary in synchronous rotation.We have modelled this process by adapting a method that has proved successful in explaining the properties of the intermediate polars (Wynn & King 1995). We model the secondary magnetic field as a pure dipole which drags on the wind material as it crosses field lines. This has the effect of trying to force material to move along field lines. This does not model the magnetic interaction completely since it does not allow for the back reaction of the material on the field structure. However, this will only become significant in regions where the field is weak and hence where material is not exerting a large torque on the star.