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

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 ∼ 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.

A catalogue of white dwarfs in Gaia EDR3

“…Cheng et al (2019) later demonstrated that about 6 per cent of high-mass white dwarfs ( > 1.05 M ⊙ ) on this transverse sequence, likely the products of double-degenerate mergers, must experience an extra 8 Gyr cooling delay not explained by core-crystallization alone. More recently, Blouin et al (2021) reconciled these results showing that a distillation process during 22 Ne phase separation in crystallising white dwarfs could explain both the cooling delay of standard white dwarfs and the extra delay experienced by high-mass double white dwarf mergers (see also Bauer et al 2020;Camisassa et al 2021). A number of additional studies have focused on the spectral properties of ultra-massive white dwarfs, consolidating the idea that many of these systems are the result of double white dwarf mergers (Hollands et al 2020;Kawka et al 2020;Kilic et al 2021).…”

Direct Evaluation of the Phase Diagrams of Dense Multicomponent Plasmas by Integration of the Clapeyron Equations

“…Note that the integrations at different T are independent from one another and can be performed simultaneously. We have successfully applied this approach to the three-component C/O/Ne mixture found in white dwarf cores [53].…”