Secondary star magnetic fields in close binaries

Abstract

In this paper an attempt is made to use magnetic braking to constrain the surface polar field strengths of the secondary stars in close binaries with orbital periods between P orb ∼ 3 and 7 h. By using a standard Mestel & Spruit model, assuming field saturation, a linear relation was obtained between the mass transfer and orbital period, for the period range under consideration, which allows constraining the surface polar field between B o,2 ∼ 2900 and 3300 G. It has been shown that the predicted mass-transfer rates correlate with the Verbunt & Zwaan empirical mass transfer versus orbital period relation. Furthermore, it has been shown that the closed field lines, that is, the so-called dead zone, of the secondary star envelopes a substantial fraction of the binary, including the white dwarf, for all orbital periods under consideration. It has been shown that the interaction of the white dwarf field with the surrounding envelope can result in the intermediate polars entering the period gap being magnetically synchronized over time-scales τ syn ≤ 10 7 yr. This mechanism may then play a significant role in the orbital evolution of the intermediate polars into the polars. Furthermore, surface magnetic field structures in the L 1 region may play a significant role in the fragmentation of the mass flow near the L1 region, which may explain the inferred fragmented mass transfer, that is, the blobby mass flow, in magnetic cataclysmic variables.