Hot DA white dwarf model atmosphere calculations: Including improved Ni PI cross sections

TL;DR

This study improves white dwarf atmosphere models by incorporating more accurate Ni atomic data, revealing significant effects on EUV flux and metal abundance measurements, especially for nitrogen and oxygen.

Contribution

The paper introduces enhanced Ni atomic data into white dwarf models, demonstrating substantial impacts on EUV flux and element abundance determinations compared to previous models.

Findings

Significant EUV flux attenuation with new Ni data.

Altered N and O abundance estimates by up to 22%.

Major changes in Ni abundance when using extended line lists.

Abstract

To calculate realistic models of objects with Ni in their atmospheres, accurate atomic data for the relevant ionization stages needs to be included in model atmosphere calculations. In the context of white dwarf stars, we investigate the effect of changing the Ni {\sc iv}-{\sc vi} bound-bound and bound-free atomic data has on model atmosphere calculations. Models including PICS calculated with {\sc autostructure} show significant flux attenuation of up to $\sim 80$\% shortward of 180\AA\, in the EUV region compared to a model using hydrogenic PICS. Comparatively, models including a larger set of Ni transitions left the EUV, UV, and optical continua unaffected. We use models calculated with permutations of this atomic data to test for potential changes to measured metal abundances of the hot DA white dwarf G191-B2B. Models including {\sc autostructure} PICS were found to change the abundances of N and O by as much as $\sim 22$\% compared to models using hydrogenic PICS, but heavier species were relatively unaffected. Models including {\sc autostructure} PICS caused the abundances of N/O {\sc iv} and {\sc v} to diverge. This is because the increased opacity in the {\sc autostructure} PICS model causes these charge states to form higher in the atmosphere, moreso for N/O {\sc v}. Models using an extended line list caused significant changes to the Ni {\sc iv}-{\sc v} abundances. While both PICS and an extended line list cause changes in both synthetic spectra and measured abundances, the biggest changes are caused by using {\sc autostructure} PICS for Ni.