Constraining the origin of magnetic white dwarfs

The origin of magnetic white dwarfs (MWDs) has been a long-standing puzzle. Proposed origin mechanisms have included: fossil fields frozen in from the progenitor convective core; a dynamo in the progenitor envelope; crystallization dynamos in sufficiently cool white dwarfs; and accretion disk dynamos from white dwarf-white dwarf mergers or tidally shredded low-mass stellar or planetary companions. Here we show how observational constraints on white dwarf magnetic field strengths, ages, and masses can be used to narrow down the viability of proposed origin mechanisms. Using data from both the Montreal White Dwarf Database and a 20 pc volume-limited sample from Gaia DR2, we find that the fossil field mechanism overpredicts the number of magnetic white dwarfs, which suggests, as supported by theoretical arguments, that the field is not actually frozen into the progenitor cores but diffuses before the white dwarf forms. Crystallization dynamos, occur too late to explain the bulk of magnetic white dwarfs. And with the progenitor envelope dynamos impeded by the theoretical challenge of depositing a field from envelope to white dwarf core, the two disk dynamo mechanisms emerge as the field origin mechanisms most resilient to present constraints, with mergers best able to explain the high mass, strongly magnetized young MWDs. The methods herein also reveal observational data gaps, and motivate future acquisition of more complete data.