Origin of strong magnetic fields in white dwarfs
In plain words
Some white dwarfs carry magnetic fields millions of times stronger than the Sun's, and they become more common as white dwarfs age. It is unknown whether the fields are inherited from the parent star, produced in mergers, or generated when the white dwarf's core freezes.
Precise statement
Volume-limited samples show the magnetic fraction of white dwarfs rising with cooling age, with fields from about 1e3 to 1e9 G. Determine which mechanism (fossil field from the progenitor, dynamo during a merger or common envelope, or a dynamo driven by compositional convection during core crystallization) accounts for the observed incidence versus age, mass and binarity and for the field-strength distribution, including the breakout time of a field generated deep in the core.
What would settle it
Dynamo simulations of crystallizing and merging white dwarfs that predict field strength and surface emergence time, compared with spectropolarimetric volume-limited samples.
Status in the literature
Unverified note
2024 work found the crystallization dynamo is short and intense and that field breakout can be delayed by long diffusion times (Fuentes et al.; Castro-Tapia et al., 2024), leaving the dominant channel unsettled.