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

Abstract

We discuss the importance of pure hydrogen white dwarf atmosphere models with Ly-$\alpha$ 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-$\alpha$ 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 $T_{\mathrm{eff}}<6000\mathrm{K}$ 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 $\sim 160K$ cooler when models that account for Ly-$\alpha$ 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-$\alpha$ opacity for precise dating of old stellar populations from white dwarf cooling sequences.

4 pages, 4 figures. Accepted for publication in A&A

References in corpus (3)

• Found: The missing blue opacity in atmosphere models of cool hydrogen white dwarfs
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• The Physics of Crystallization from Globular Cluster White Dwarf Stars in NGC 6397
• A Nearby Old Halo White Dwarf Candidate from the Sloan Digital Sky Survey
• Infrared absorption of dense helium and its importance in the atmospheres of cool white dwarfs
• Near-UV absorption in very cool DA white dwarfs
• Complex hydrogen chemical equilibrium and Gaia low mass problem in cool white dwarfs