White dwarf atmosphere models with Ly-$\alpha$ opacity in the analysis of the white dwarf cooling sequence of NGC 6397

TL;DR

This paper emphasizes the importance of incorporating Ly-$\alpha$ opacity in white dwarf atmosphere models to accurately analyze the cooling sequence and age of the globular cluster NGC 6397, leading to more precise stellar dating.

Contribution

The authors present improved white dwarf atmosphere models that include Ly-$\alpha$ opacity, significantly enhancing the accuracy of age estimates for old stellar populations.

Findings

Models with Ly-$\alpha$ opacity reproduce UV colors of cool white dwarfs.

Including Ly-$\alpha$ opacity increases the estimated white dwarf temperatures by about 160 K.

Using these models adjusts the cluster age estimate by approximately 0.5 Gyr.

Abstract

We discuss the importance of pure hydrogen white dwarf atmosphere models with Ly-$\rm \alpha$ far red wing opacity in the analysis of the white dwarf cooling sequence of the globular cluster NGC 6397. Our recently improved atmosphere models account for the previously missing opacity from the Ly-$\rm \alpha$ hydrogen line broadened by collisions of the absorbing hydrogen atoms with molecular and atomic hydrogen. These models are the first that well reproduce the UV colors and spectral energy distributions of cool white dwarfs with $T_{\rm eff}<6000 \rm K$ observed in the Galactic Disk. Fitting the observed $F814W$ magnitude and $F606W-F814W$ color we obtained a value for the true distance modulus, $\mu=12.00 \pm 0.02$, that is in agreement with recent analyses. We show that the stars at the end of the cooling sequence appear to be $\rm \sim 160 K$ cooler when models that account for Ly-$\rm \alpha$ opacity are used. This indicates that the age of NGC 6397 derived from the white dwarf cooling sequence using atmosphere models that do not include the correct Ly-$\alpha$ opacity is underestimated by $\sim 0.5$ Gyr. Our analysis shows that it is essential to use white dwarf atmosphere models with Ly-$\rm \alpha$ opacity for precise dating of old stellar populations from white dwarf cooling sequences.