Phase Separation in Ultramassive White Dwarfs

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