Direct MD simulation of liquid-solid phase equilibria for three-component plasma

TL;DR

This study uses molecular dynamics simulations to investigate how 22Ne impurities affect liquid-solid phase equilibria in carbon-oxygen-neon plasmas, relevant for white dwarf star cooling, revealing minimal impact of neon but discrepancies in melting temperatures.

Contribution

The paper provides the first MD simulation analysis of three-component plasma phase equilibria, highlighting limitations of existing semi-analytic models for multicomponent systems.

Findings

Neon impurities do not significantly alter chemical separation in white dwarf matter.

MD simulations show systematically lower melting temperatures than semi-analytic models.

Discrepancies increase with impurity parameter, indicating issues with current free energy correction methods.

Abstract

The neutron rich isotope 22Ne may be a significant impurity in carbon and oxygen white dwarfs and could impact how the stars freeze. We perform molecular dynamics simulations to determine the influence of 22Ne in carbon-oxygen-neon systems on liquid-solid phase equilibria. Both liquid and solid phases are present simultaneously in our simulation volumes. We identify liquid, solid, and interface regions in our simulations using a bond angle metric. In general we find good agreement for the composition of liquid and solid phases between our MD simulations and the semi analytic model of Medin and Cumming. The trace presence of a third component, neon, does not appear to strongly impact the chemical separation found previously for two component carbon and oxygen systems. This suggests that small amounts of 22Ne may not qualitatively change how the material in white dwarf stars freezes. However, we do find systematically lower melting temperatures (higher Gamma) in our MD simulations compared to the semi analytic model. This difference seems to grow with impurity parameter Q_imp and suggests a problem with simple corrections to the linear mixing rule for the free energy of multicomponent solid mixtures that is used in the semi analytic model.