Secondary star surface magnetic activity and mass transfer in cataclysmic variables

Abstract

In this study it is shown that secondary star magnetic fields influence the mass transfer process in close interacting binaries, especially cataclysmic variables (CVs) and thus play a fundamental role in the whole mass transfer process, and evolution of these systems. The Mestel and Spruit (1987) stellar wind theory is used to model the surface magnetic field of the secondary star in CVs, particularly the intermediate polars, constraining the angular momentum that is required to drive the observed mass transfer rate through Roche lobe overflow. This in turn allows solving for the mass transfer rates, via magnetic braking, and the surface polar magnetic field of these stars. These field strengths are used to study and constrain magnetic advection from the secondary star to the primary star, and its effect on the mass flow in the funnel in magnetic CVs. This has important consequences for the so-called magnetic viscosity in the accretion discs of disc accreting magnetic cataclysmic variables, which are fed by these magnetic secondary stars. It is shown that the mass transfer rates in these systems vary with orbital period, with lower mass transfer rates in more compact systems than in the wider systems. It is also shown that advection of magnetic flux into the funnel results in severe magnetic viscosity at the L1 region. The advected magnetic field into the funnel flow results in a magnetized flow and enhanced magnetic pressure in the L1 region. Since the magnetic pressure in the L1 region exceeds the flow ram pressure, continuous flow of material through the L1 region is prevented. It is shown that matter can easily cross the funnel if pressure builds up behind the barrier. This therefore implies that the mass transfer in these systems is not continuous but fragmented in the form of blobs.