A possible solution to the Lyman/Balmer line problem in hot DA white dwarfs

TL;DR

This paper investigates the Lyman/Balmer line discrepancy in hot DA white dwarfs by examining atomic data, broadening treatments, and atmospheric composition, aiming to improve temperature determinations.

Contribution

It introduces a systematic analysis of model physics variables affecting the Lyman/Balmer line discrepancy in hot white dwarfs, with preliminary results on effective temperature.

Findings

Discrepancies increase with temperature above 50,000K.

Different atomic data and broadening treatments impact temperature estimates.

Metallicity variations are planned for future analysis.

Abstract

Arguably, the best method for determining the effective temperature ($T_{\mathrm{eff}}$) and surface gravity (log $g$) of a DA white dwarf is by fitting the Hydrogen Lyman and Balmer absorption features. However, as has been shown for white dwarfs with $T_{\mathrm{eff}}$>50,000K, the calculated value from the Lyman and Balmer lines are discrepant, which worsens with increasing temperature. Many different solutions have been suggested, ranging from the input physics used to calculate the models, to interstellar reddening. We will focus on the former, and consider three variables. The first is the atomic data used, namely the number of transitions included in line blanketing treatments and the photoionization cross sections. The second is the stark broadening treatment used to synthesise the Lyman and Balmer line profiles, namely the calculations performed by Lemke (1997) and Tremblay & Bergeron (2009). Finally, the third is the atmospheric content. The model grids are calculated with a pure H composition, and a metal polluted composition using the abundances of Preval et al. (2013). We present the preliminary results of our analysis, whereby we have determined the $T_{\mathrm{eff}}$ for a small selection of white dwarfs. We plan to extend our analysis by allowing metallicity to vary in future model grids.