Fluid Mixing during Phase Separation in Crystallizing White Dwarfs

TL;DR

This paper investigates fluid mixing during phase separation in crystallizing white dwarfs, showing thermohaline mixing is sufficient for chemical redistribution but unlikely to generate magnetic fields, and impacts pulsation frequencies.

Contribution

It demonstrates that thermohaline mixing can explain chemical redistribution in white dwarf cores and challenges previous assumptions about turbulent mixing and magnetic field generation.

Findings

Thermohaline mixing is efficient enough for chemical redistribution.

Reduced fluid velocities cannot generate measurable magnetic fields.

Mixing alters chemical profiles affecting pulsation mode frequencies.

Abstract

Accurate models of cooling white dwarfs must treat the energy released as their cores crystallize. This phase transition slows the cooling by releasing latent heat and also gravitational energy, which results from phase separation: liquid C is released from the solid C/O core, driving an outward carbon flux. The Gaia color-magnitude diagram provides striking confirmation of this theory by revealing a mass-dependent overdensity of white dwarfs, indicating slowed cooling at the expected location. However, the observed overdensity is enhanced relative to the models. Additionally, it is associated with increased magnetism, suggesting a link between crystallization and magnetic field generation. Recent works aimed at explaining an enhanced cooling delay and magnetic field generation employ a uniform mixing prescription that assumes large-scale turbulent motions; we show here that these calculations are not self-consistent. We also show that thermohaline mixing is most likely efficient enough to provide the required chemical redistribution during C/O phase separation, and that the resulting velocities and mixing lengths are much smaller than previous estimates. These reduced fluid motions cannot generate measurable magnetic fields, suggesting any link with crystallization needs to invoke a separate mechanism. Finally, this mixing alters the chemical profiles which in turn affects the frequencies of the pulsation modes.