Crystallization of Carbon Oxygen Mixtures in White Dwarf Stars

TL;DR

This study uses molecular dynamics simulations to determine the phase diagram of dense carbon/oxygen mixtures in white dwarf stars, providing insights into their crystallization temperatures and core compositions.

Contribution

It presents a new phase diagram for carbon/oxygen mixtures in white dwarf interiors, aligning with some prior models and offering constraints on stellar core compositions.

Findings

Phase diagram agrees with Ogata et al. and Medin and Cumming

Lower melting temperatures than Segretain et al.

Suggests central oxygen abundance in WDs is less than 60%

Abstract

We determine the phase diagram for dense carbon/ oxygen mixtures in White Dwarf (WD) star interiors using molecular dynamics simulations involving liquid and solid phases. Our phase diagram agrees well with predictions from Ogata et al. and Medin and Cumming and gives lower melting temperatures than Segretain et al. Observations of WD crystallization in the globular cluster NGC 6397 by Winget et al. suggest that the melting temperature of WD cores is close to that for pure carbon. If this is true, our phase diagram implies that the central oxygen abundance in these stars is less than about 60%. This constraint, along with assumptions about convection in stellar evolution models, limits the effective S factor for the $^{12}$C($\alpha,\gamma$)$^{16}$O reaction to S_{300} <= 170 keV barns.