Comprehensive abundance analysis of red giants in the open clusters Stock 2, NGC 2168, 6475, 6991 and 7762. (arXiv:1902.02939v1 [astro-ph.SR])

Comprehensive abundance analysis of red giants in the open clusters Stock 2, NGC 2168, 6475, 6991 and 7762. (arXiv:1902.02939v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Reddy_A/0/1/0/all/0/1">Arumalla B. S. Reddy</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Lambert_D/0/1/0/all/0/1">David L. Lambert</a> (1) ((1) W.J. McDonald Observatory and Department of Astronomy, The University of Texas at Austin, USA.)

We have analysed high-dispersion echelle spectra ($R = 60000$) of red giant
members of five open clusters to derive abundances for many elements from Na to
Eu. The [Fe/H] values are $-0.06pm0.03$ for Stock 2, $-0.11pm0.03$ for NGC
2168, $-0.01pm0.03$ for NGC 6475, $0.00pm0.03$ for NGC 6991 and
$-0.07pm0.03$ for NGC 7662. Sodium is enriched in the giants relative to the
abundance expected of main sequence stars of the same metallicity. This
enrichment of [Na/Fe] by about $+0.25$ attributed to the first dredge-up is
discussed in the light of theoretical predictions and recently published
abundance determinations. Abundance ratios [El/Fe] for other elements are with
very few exceptions equal to those of field giants and dwarfs, i.e., [El/Fe]
$simeq 0.00$ for [Fe/H] $sim 0.0$. An exception is the overabundance of La,
Ce, Nd and Sm in NGC 6991 but this is consistent with our previous
demonstration that the abundances of these $s$-process products vary by about
$pm0.2$ among clusters of the same [Fe/H], a variation found also among field
giants and dwarfs.

We have analysed high-dispersion echelle spectra ($R = 60000$) of red giant
members of five open clusters to derive abundances for many elements from Na to
Eu. The [Fe/H] values are $-0.06pm0.03$ for Stock 2, $-0.11pm0.03$ for NGC
2168, $-0.01pm0.03$ for NGC 6475, $0.00pm0.03$ for NGC 6991 and
$-0.07pm0.03$ for NGC 7662. Sodium is enriched in the giants relative to the
abundance expected of main sequence stars of the same metallicity. This
enrichment of [Na/Fe] by about $+0.25$ attributed to the first dredge-up is
discussed in the light of theoretical predictions and recently published
abundance determinations. Abundance ratios [El/Fe] for other elements are with
very few exceptions equal to those of field giants and dwarfs, i.e., [El/Fe]
$simeq 0.00$ for [Fe/H] $sim 0.0$. An exception is the overabundance of La,
Ce, Nd and Sm in NGC 6991 but this is consistent with our previous
demonstration that the abundances of these $s$-process products vary by about
$pm0.2$ among clusters of the same [Fe/H], a variation found also among field
giants and dwarfs.

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