Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: White dwarf atmosphere models with Ly-$\alpha$ opacity in the analysis
  of the white dwarf cooling sequence of NGC 6397   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.

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The categories of this paper are: dg-ga
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The categories of this paper are: stat
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The categories of this paper are: atom-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: math-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: q-fin
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The categories of this paper are: physics
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The categories of this paper are: solv-int
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The categories of this paper are: gr-qc
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The categories of this paper are: quant-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: alg-geom
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The categories of this paper are: hep-lat
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The categories of this paper are: nucl-ex