Friday, April 18, 2025 at 12:15 pm

JILA Foothills Room

Matias Castro-Tapia , McGill University

"Cool White Dwarf Stars: Crystallization, Convection, and Magnetic Fields "

Abstract:

A crystallization-driven dynamo from compositional convection has been proposed to explain strong magnetic fields (≥1 MG) in isolated white dwarfs (WDs), but its efficiency remains debated. In this work, we show that the dynamo is only feasible for a few million years near the onset of crystallization. Using theory and simulations, we identify two convection regimes: slow, inefficient thermohaline convection and fast, efficient compositionally-driven convection. We found that only the fast regime can generate strong magnetic fields (1–100 MG), and whether these fields can emerge at the surface depends on whether there is enough time for outwards transport before the star crystallizes. We show the most detailed theoretical analysis of the crystallization-driven dynamo to date and conclude that just a few observed magnetic WDs can be explained by this theory.

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