Diffusion of Neon in White Dwarf Stars

TL;DR

This paper calculates the self-diffusion constants of neon in white dwarf plasma mixtures using molecular dynamics, showing that these constants are well-understood and have minimal impact on white dwarf cooling models.

Contribution

It provides accurate diffusion constants for neon in plasma mixtures, improving the modeling of white dwarf cooling processes.

Findings

Diffusion constants are similar to one-component plasma results.

D_i scales as Z_i^{-2/3} at strong coupling.

Uncertainties in D_Ne are negligible for white dwarf simulations.

Abstract

Sedimentation of the neutron rich isotope $^{22}$Ne may be an important source of gravitational energy during the cooling of white dwarf stars. This depends on the diffusion constant for $^{22}$Ne in strongly coupled plasma mixtures. We calculate self-diffusion constants $D_i$ from molecular dynamics simulations of carbon, oxygen, and neon mixtures. We find that $D_i$ in a mixture does not differ greatly from earlier one component plasma results. For strong coupling (coulomb parameter $\Gamma>$ few), $D_i$ has a modest dependence on the charge $Z_i$ of the ion species, $D_i \propto Z_i^{-2/3}$. However $D_i$ depends more strongly on $Z_i$ for weak coupling (smaller $\Gamma$). We conclude that the self-diffusion constant $D_{\rm Ne}$ for $^{22}$Ne in carbon, oxygen, and neon plasma mixtures is accurately known so that uncertainties in $D_{\rm Ne}$ should be unimportant for simulations of white dwarf cooling.