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