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.

arXiv comment4 pages, 4 figures. Accepted for publication in A&A

References in corpus (14)

• The Photometric Performance and Calibration of the HST Advanced Camera For Surveys
• Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements
• Distances and ages of NGC 6397, NGC 6752 and 47 Tuc
• Found: The missing blue opacity in atmosphere models of cool hydrogen white dwarfs
• The White Dwarf Cooling Sequence of the Globular Cluster Messier 4
• The White Dwarf Luminosity Function from SDSS Imaging Data
• Cooling Models for Old White Dwarfs
• Debris Disks Around White Dwarfs: The DAZ Connection
• HST Observations of the White Dwarf Cooling Sequence of M4
• Pure hydrogen atmosphere for very cool white dwarfs
• Probing the Faintest Stars in a Globular Star Cluster
• Isochrones and Luminosity Functions for Old White Dwarfs
• The Halo White Dwarf WD 0346+246 Revisited
• Concerning the White Dwarf Cooling Age of M4: A Response to the Paper by De Marchi et al. on "A Different Interpretation of Recent HST Observations"

Cited by in corpus (13)

• A large stellar evolution database for population synthesis studies: VI. White dwarf cooling sequences
• The Masses of Population II White Dwarfs
• The End of the White Dwarf Cooling Sequence in M4: an efficient approach
• The Physics of Crystallization from Globular Cluster White Dwarf Stars in NGC 6397
• A comparative analysis of the observed white dwarf cooling sequence from globular clusters
• Visitors from the Halo: 11 Gyr old White Dwarfs in the Solar Neighborhood
• A Nearby Old Halo White Dwarf Candidate from the Sloan Digital Sky Survey
• Lyman-alpha wing absorption in cool white dwarf stars
• Star clusters as laboratories for stellar and dynamical evolution
• A differential chemical element analysis of the metal poor Globular Cluster NGC 6397
• 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