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The precise astrometric measurements of the Gaia Data Release 2 have openedthe 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 mustexperience an extra cooling delay of $\sim 8\,$Gyr not predicted by currentmodels. $^{22}$Ne phase separation in a crystallizing C/O white dwarf can leadto a distillation process that efficiently transports $^{22}$Ne toward itscenter, thereby releasing a considerable amount of gravitational energy. Usingstate-of-the-art Monte Carlo simulations, we show that this mechanism canlargely resolve the ultramassive cooling anomaly if the delayed populationconsists of white dwarfs with moderately above-average $^{22}$Ne abundances. Wealso argue that $^{22}$Ne phase separation can account for the smaller coolingdelay currently missing for models of white dwarfs with more standardcompositions.

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