Phase Separation in Ultramassive White Dwarfs

TL;DR

This paper investigates the crystallization and phase separation in ultramassive white dwarf cores, revealing that sodium impurities do not significantly alter core composition and providing accurate phase diagrams for modeling.

Contribution

The study introduces a Clapeyron integration technique to compute precise phase diagrams for O/Ne white dwarf mixtures, clarifying impurity effects and offering analytic fits for stellar evolution models.

Findings

Na impurities do not enrich cores via phase separation.

Impurities have negligible impact on the O/Ne phase diagram.

Provided analytic fits for white dwarf modeling.

Abstract

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 $\approx 10\%$ 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.