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Abstract: Angular momentum loss in ultracompact binaries, such as the AM CanumVenaticorum stars, is usually assumed to be due entirely to gravitationalradiation. Motivated by the outflows observed in ultracompact binaries, weinvestigate whether magnetically coupled winds could in fact lead tosubstantial additional angular momentum losses. We remark that the scalingrelations often invoked for the relative importance of gravitational andmagnetic braking do not apply, and instead use simple non-empirical expressionsfor the braking rates. In order to remove significant angular momentum, thewind must be tied to field lines anchored in one of the binary-s componentstars; uncertainties remain as to the driving mechanism for such a wind. In thecase of white dwarf accretors, we find that magnetic braking can potentiallyremove angular momentum on comparable or even shorter timescales thangravitational waves over a large range in orbital period. We present such asolution for the 17-minute binary AM CVn itself which admits a cold white dwarfdonor and requires that the accretor have surface field strength ~6E4 G. Such afield would not substantially disturb the accretion disk. Although thetreatment in this paper is necessarily simplified, and many conditions must bemet in order for a wind to operate as proposed, it is clear that magneticbraking cannot easily be ruled out as an important angular momentum sink. Wefinish by highlighting observational tests that in the next few years willallow an assessment of the importance of magnetic braking.

Author: Alison Farmer, Gijs Roelofs


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