$^{22}$Ne Phase Separation As A Solution To The Ultramassive White Dwarf Cooling Anomaly

TL;DR

This paper proposes that $^{22}$Ne phase separation during crystallization in ultramassive white dwarfs releases gravitational energy, potentially resolving observed discrepancies between white dwarf cooling models and Gaia data.

Contribution

It introduces a novel mechanism involving $^{22}$Ne phase separation that explains the ultramassive white dwarf cooling anomaly, supported by advanced Monte Carlo simulations.

Findings

$^{22}$Ne phase separation can significantly delay white dwarf cooling.

The mechanism explains the observed cooling delay in ultramassive white dwarfs.

It also accounts for smaller delays in standard composition white dwarfs.

Abstract

The precise astrometric measurements of the Gaia Data Release 2 have opened the door to detailed tests of the predictions of white dwarf cooling models. Significant discrepancies between theory and observations have been identified, the most striking affecting ultramassive white dwarfs. Cheng et al. (2019) found that a small fraction of white dwarfs on the so-called Q branch must experience an extra cooling delay of $\sim 8\,$Gyr not predicted by current models. $^{22}$Ne phase separation in a crystallizing C/O white dwarf can lead to a distillation process that efficiently transports $^{22}$Ne toward its center, thereby releasing a considerable amount of gravitational energy. Using state-of-the-art Monte Carlo simulations, we show that this mechanism can largely resolve the ultramassive cooling anomaly if the delayed population consists of white dwarfs with moderately above-average $^{22}$Ne abundances. We also argue that $^{22}$Ne phase separation can account for the smaller cooling delay currently missing for models of white dwarfs with more standard compositions.