${}^{22}$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 $\sim 8$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.

8 pages, 3 figures, 1 table. Accepted for publication in ApJL on 2021-03-23

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Cited by in corpus (19)

• A catalogue of white dwarfs in Gaia EDR3
• Current challenges in the physics of white dwarf stars
• Spectral classification of the 100 pc white dwarf population from Gaia-DR3 and the Virtual Observatory
• The spectral evolution of white dwarfs: where do we stand?
• White dwarfs as Physics laboratories: lights and shadows
• Direct Evaluation of the Phase Diagrams of Dense Multicomponent Plasmas by Integration of the Clapeyron Equations
• Formation of ultra-massive carbon-oxygen white dwarfs from the merger of carbon-oxygen and helium white dwarf pairs
• The evolution of ultra-massive carbon oxygen white dwarfs
• Carbon dredge-up required to explain the Gaia white dwarf colour-magnitude bifurcation
• Cooling Delays from Iron Sedimentation and Iron Inner Cores in White Dwarfs
• Phase Separation in Ultramassive White Dwarfs
• The non-explosive stellar merging origin of the ultra-massive carbon-rich white dwarfs
• Accurate Diffusion Coefficients for Dense White Dwarf Plasma Mixtures
• Cooling Models for the Most Massive White Dwarfs
• Distillation of ${}^{56}$Fe in Ultramassive O-Ne White Dwarfs
• The velocity distribution of white dwarfs in Gaia EDR3
• Phase diagrams of binary ionic mixtures and white dwarf cooling
• An emerging and enigmatic spectral class of isolated DAe white dwarfs
• On excess entropy and latent heat in crystallizing white dwarfs