Phase Separation in Ultramassive White Dwarfs

• Published in: The Astrophysical journal Vol. 919; no. 2; pp. 87 - 94
• Main Authors: Blouin, Simon, Daligault, Jérôme
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.10.2021; IOP Publishing
• Subjects: ASTRONOMY AND ASTROPHYSICS; Astrophysics; Cores; Crystallization; Degenerate matter; Distillation; Impurities; Phase diagrams; Phase separation; Plasma physics; Stellar evolution; Stellar interiors; Trace elements; White dwarf stars
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac1513

Ultramassive white dwarfs are extreme endpoints of stellar evolution. Recent findings, such as a missing multi-Gyr cooling delay for a number of ultramassive white dwarfs and a white dwarf with a quasi-Chandrasekhar mass, motivate a better understanding of their evolution. A key process still subject to important uncertainties is the crystallization of their dense cores, which are generally assumed to be constituted of 16 O, 20 Ne, and a mixture of several trace elements (most notably 23 Na and 24 Mg). In this work, we use our recently developed Clapeyron integration technique to compute accurate phase diagrams of three-component mixtures relevant to the modeling of O/Ne ultramassive white dwarfs. We show that, unlike the phase separation of 22 Ne impurities in C/O cores, the phase separation of 23 Na impurities in O/Ne white dwarfs cannot lead to the enrichment of their cores in 23 Na via a distillation process. This severely limits the prospect of transporting large quantities of 23 Na toward the center of the star, as needed in the white dwarf core-collapse mechanism recently proposed by Caiazzo et al. We also show that despite representing ≈10% of the ionic mixture, 23 Na and 24 Mg impurities only have a negligible impact on the O/Ne phase diagram, and the two-component O/Ne phase diagram can be safely used in white dwarf evolution codes. We provide analytic fits to our high-accuracy O/Ne phase diagram for implementation in white dwarf models.