The Free-Free Opacity in Warm, Dense, and Weakly Ionized Helium

TL;DR

This paper explores the ionization and opacity of warm, dense helium relevant to cool white dwarf atmospheres, comparing models and simulations to improve understanding of weak ionization conditions.

Contribution

It introduces a comprehensive analysis combining non-ideal chemical models, QMD simulations, and experimental data to better estimate helium ionization in dense, warm conditions.

Findings

Ionization fractions vary greatly across models.

Density estimates for helium pressure ionization range from 0.3 to 14 g/cm³.

New insights into electron-atom inverse bremsstrahlung absorption in helium.

Abstract

We investigate the ionization and the opacity of warm, dense helium under conditions found in the atmospheres of cool white dwarf stars. Our particular interest is in densities up to $\rm 3 g/cm^{3}$ and temperatures from 1000K to 10000K. For these physical conditions various approaches for modeling the ionization equilibrium predict ionization fractions that differ by orders of magnitudes. Furthermore, estimates of the density at which helium pressure-ionizes vary from $\rm 0.3$ to $\rm 14 g/cm^{3}$. In this context, the value of the electron-atom inverse bremsstrahlung absorption is highly uncertain. We present new results obtained from a non-ideal chemical model for the ionization equilibrium, from Quantum Molecular Dynamics (QMD) simulations, and from the analysis of experimental data to better understand the ionization fraction in fluid helium in the weak ionization limit.