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 .
The downward pumping of angular momentum in the convective envelope , in combination with the absorption of a planet or tidal spin-up from a binary companion , can trigger strong dynamo action near the core-envelope boundary .
Several arguments point to the outer core as the source for a magnetic field in the WD remnant : the outer third of a \sim 0.55 M _ { \odot } WD is processed during the shell burning phase ( s ) of the progenitor ; the 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 the intense radiation flux at the core boundary facilitates magnetic buoyancy within a relatively thick tachocline layer .
The helicity flux into the growing core is driven by a dynamical imbalance with a latitude-dependent rotational stress .
The magnetic field deposited in an isolated massive WD is concentrated in an outer shell of mass \lesssim 0.1 M _ { \odot } and can reach \sim 10 MG. A buried toroidal field experiences moderate ohmic decay above an age \sim 0.3 Gyr , which may lead to growth or decay of the external magnetic field .
The final WD spin period is related to a critical spin rate below which magnetic activity shuts off , and core and envelope decouple ; it generally sits in the range of hours to days .
WD periods ranging up to a year are possible if the envelope re-expands following a late thermal pulse .