A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs

Abstract

The origin of large magnetic fields (≳106 G) in isolated white dwarfs is not clear. One possible explanation is that crystallization of the star’s core drives compositional convection, which when combined with the star’s rotation, can drive a dynamo. However, whether convection is efficient enough to explain the large intensity of the observed magnetic fields is still under debate. Recent work has shown that convection in cooling white dwarfs spans two regimes: efficient convection at the onset of crystallization, and thermohaline convection during most of the star’s cooling history. Here, we calculate the properties of crystallization-driven convection for cooling models of several white dwarfs of different masses. We combine mixing-length theory with scalings from magnetorotational convection to estimate the typical magnitude of the convective velocity and induced magnetic field for both scenarios. In the thermohaline regime, we find velocities ∼10−6–10−5 cm s−1, with fields restricted to ≲ 100 G. However, when convection is efficient, the flow velocity can reach magnitudes of ∼102–103 cm s−1, with fields of ∼106–108 G, independent of the star’s rotation rate. Thus, dynamos driven at the onset of crystallization could explain the large intensity magnetic fields measured for single white dwarfs.

Fast and Slow Crystallization-driven Convection in White Dwarfs

“…However, it is intriguing that the convective velocities we find in the very early phases of crystallization appear to be large enough to generate magnetic fields comparable to those observed. We explore the possibility of a short-lived intense dynamo following the onset of crystallization and the expected magnetic field strengths taking into account rotation in a companion paper (Fuentes et al 2024) Our calculations can be improved in several respects. The white dwarf cooling models that we use include chemical separation following Bauer (2023), in which it is assumed that the fluid layer ahead of the crystallization front is fully mixed, and the gravitational energy release from chemical separation is deposited by hand in the model.…”

“…In this case, we used a time step of ∼10 5 yr in the early stages for all the models, allowing us to solve an efficient regime that lasts only a few Myr. While we do not attempt to show how small the core could be, in a companion paper, Fuentes et al (2024), we checked that using an analytical estimation for τ also predicts an efficient convection regime that occurs during ∼10 Myr from the onset of crystallization.…”