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.
Highlights
The presence of metals in white dwarf (WD) atmospheres can have dramatic effects on both the structure of the atmosphere, and the observed effective temperature (Teff)
Reaching a maximum of ∼80 per cent with respect to Model 1. This is indicative of a larger opacity due to the AUTOSTRUCTURE photoionization cross-section (PICS) for Ni
Using Model 1, we measured an abundance of 1.47+−00..0077 × 10−6, whereas for Models 3 and 4 we measured abundances of 1.69+−00..0088 × 10−6 and 1.79+−00..0088 × 10−6, respectively. This corresponds to an increase of ∼15 per cent and ∼22 per cent for Models 3 and 4 over Model 1, respectively. These results suggest that the largest changes to the O IV–V may be caused by the PICS rather than the number of transitions included in the line list
Summary
The presence of metals in white dwarf (WD) atmospheres can have dramatic effects on both the structure of the atmosphere, and the observed effective temperature (Teff). These effects have been demonstrated convincingly by Barstow, Hubeny & Holberg (1998). The authors determined the Teff and surface gravity (log g) of several hot DA WDs using a set of model atmosphere grids, which were either pure H and He, or heavy metal polluted. It was found that the Teff determined using the pure H model grid was ≈4000–7000 K higher than if a heavy metal-polluted model grid were used. A study by Chayer, Fontaine & Wesemael (1995)
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