Stellar Magnetic Winds
The tidally influenced secondary stars in interacting binaries are rapidly rotating and possess convective envelopes or are fully convective. This makes the generation of large-scale magnetic fields in such stars very likely, and magnetically influenced wind flows from the stellar surface would then lead to magnetic braking. The secondary would be spun down to an under-synchronous state and tides would then operate to spin the star up at the expense of the orbital angular momentum. This mechanism can account for the higher mass transfer rates occurring in binaries above the period gap, and hence the theory of magnetic braking has important application to interacting binaries.
The essentials of stellar magnetic braking theory are presented here, with particular emphasis on the fast rotator regime. This is applied to derive mass transfer rates in binaries with periods ≳ 3.0 h, for a range of laws relating the secondary’s surface magnetic field to its rotation rate. Explanations for the period gap, and why AM Herculis binaries appear not to be affected by the gap, are discussed.
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