Astronomy & Astrophysics

A&A 474, 491-494 (2007)
DOI: 10.1051/0004-6361:20077985

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

P. M. Kowalski

Lehrstuhl für Theoretische Chemie, Ruhr-Universität, 44780 Bochum, Germany
e-mail: piotr.kowalski@theochem.ruhr-uni-bochum.de

(Received 31 May 2007 / Accepted 27 June 2007)

Abstract
Aims. 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.
Methods.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 $T_{\rm eff}<6000$ K observed in the Galactic Disk.
Results.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 ~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 ~0.5 Gyr.
Conclusions.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.

Key words: stars: white dwarfs -- stars: atmospheres -- Galaxy: globular clusters: individual: NGC 6397

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