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 merger-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 constrain the viability of proposed origin mechanisms. Using data from both an expanded catalog of 1158 MWDs 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, that additional constraints beyond just the WD mass being contained in the progenitor convective core is required to determine which WDs retain fossil fields. Crystallization dynamos occur too late to explain the bulk of magnetic white dwarfs. 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 young, high mass, strongly magnetized MWDs. The methods herein also reveal observational data gaps and motivate future acquisition of more complete data.