22Ne Phase Separation as a Solution to the Ultramassive White Dwarf Cooling Anomaly

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. found that a small fraction of white dwarfs on the so-called Q branch must experience an extra cooling delay of similar to 8 Gyr not predicted by current models. Ne-22 phase separation in a crystallizing C/O white dwarf can lead to a distillation process that efficiently transports Ne-22 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 Ne-22 abundances. We also argue that Ne-22 phase separation can account for the smaller cooling delay currently missing for models of white dwarfs with more standard compositions.

Automated summary

The precise astrometric measurements of the Gaia Data Release 2 have revealed significant discrepancies between theory and observations for white dwarf cooling models, particularly affecting ultramassive white dwarfs. Ne-22 phase separation in a crystallizing C/O white dwarf can lead to a distillation process that efficiently transports Ne-22, releasing a considerable amount of gravitational energy. This mechanism may largely resolve the ultramassive cooling anomaly and could account for the smaller cooling delay in white dwarf models with more standard compositions.