A New Generation of Cool White Dwarf Atmosphere Models. I. Theoretical Framework and Applications to DZ Stars

TL;DR

This paper develops advanced atmosphere models for cool helium-rich white dwarfs, incorporating high-density physics to better reproduce observed spectra, especially for stars with spectral features like DZ stars.

Contribution

It introduces a revised model atmosphere code with state-of-the-art physics for dense helium environments, improving spectral fits for cool DZ white dwarfs.

Findings

Enhanced spectral fits for Ross 640 and LP 658-2

New ab initio calculations of heavy element ionization

Improved modeling of high-density stellar atmospheres

Abstract

The photospheres of the coolest helium-atmosphere white dwarfs are characterized by fluid-like densities. Under those conditions, standard approximations used in model atmosphere codes are no longer appropriate. Unfortunately, the majority of cool He-rich white dwarfs show no spectral features, giving us no opportunities to put more elaborate models to the test. In the few cases where spectral features are observed (such as in cool DQ or DZ stars), current models completely fail to reproduce the spectroscopic data, signaling shortcomings in our theoretical framework. In order to fully trust parameters derived solely from the energy distribution, it is thus important to at least succeed in reproducing the spectra of the few coolest stars exhibiting spectral features, especially since such stars possess even less extreme physical conditions due to the presence of heavy elements. In this paper, we revise every building block of our model atmosphere code in order to eliminate low-density approximations. Our updated white dwarf atmosphere code incorporates state-of-the-art constitutive physics suitable for the conditions found in cool helium-rich stars (DC and DZ white dwarfs). This includes new high-density metal line profiles, nonideal continuum opacities, an accurate equation of state and a detailed description of the ionization equilibrium. In particular, we present new ab initio calculations to assess the ionization equilibrium of heavy elements (C, Ca, Fe, Mg and Na) in a dense helium medium and show how our improved models allow us to achieve better spectral fits for two cool DZ stars, Ross 640 and LP 658-2.