Spin and Magnetism of White Dwarfs

Yevgeni Kissin, Christopher Thompson

Published 2015-01-28Version 1

The magnetism and rotation of white dwarf (WD) stars are investigated in relation to a hydromagnetic dynamo operating in the progenitor during shell burning phases. We find that the downward pumping of angular momentum in the convective envelope can, by itself, trigger dynamo action near the core-envelope boundary in an isolated intermediate-mass star. A solar-mass star must receive additional angular momentum following its rotational braking on the main sequence, either by a merger with a planet, or by tidal interaction in a stellar binary. Several arguments point to the outer core as the source for a magnetic field in the WD remnant: i) the outer third of a ~0.55$M_\odot$ WD is processed during the shell burning phases of the progenitor; ii) escape of magnetic helicity through the envelope mediates the growth of (compensating) helicity in the core, as is needed to maintain a stable magnetic field in the remnant; and iii) intense radiation flux at the core boundary facilitates magnetic buoyancy within a relatively thick tachocline layer. The helicity flux into the core is dominated by a persistent magnetic twist, which maintains solid rotation in the core against a latitude-dependent convective stress. The magnetic field deposited in an isolated massive WD can reach ~10MG, and is enhanced in strength if the star experiences an interaction with a brown dwarf or low-mass star. A buried toroidal field experiences moderate ohmic decay above an age ~1 Gyr, which may lead to growth or decay of the external magnetic field. The final WD spin period is related to a critical Coriolis parameter below which magnetic activity shuts off, and core and envelope decouple; it generally sits in the range of hours to days. A wider range of spin periods is possible when the star spins rapidly enough that core and envelope remain magnetically coupled, ranging from less than a day up to a year. (abridged)