Hyades Oxygen Abundances from the k6300 [O i] Line: The Giant-Dwarf Oxygen Descrepancy RevisitedThe Astrophysical Journal
PublisherThe American Astronomical Society
AbstractWe present the results of our abundance analysis of Fe, Ni, and O in high signal-to-noise ratio, high-resolution Very Large Telescope UVES and McDonald 2dcoude´ spectra of nine dwarfs and three giants in the Hyades open cluster. The difference in Fe abundances derived from Fe ii and Fe i lines ([Fe ii/HÀ½Fe i/H)and Ni i abundances derived from moderately high-excitation ( %4:20 eV) lines is found to increase with decreasing Teff for the dwarfs. Both of these ﬁndings are in concordance with previous results of overexcitation/overionization in cool young dwarfs. Oxygen abundances are derived from the [O i] k6300 line, with careful attention given to the Ni i blend. The dwarf O abundances are in star-to-star agreement within uncertainties, but the abundances of the three coolest dwarfs (4573 K TeA 4834 K) evince an increase with decreasing Teff. Possible causes for the apparent trend are considered,including the effects of overdissociation of O-containing molecules.O abundances are derived from the near-UV OH k3167 line in high-quality Keck HIRES spectra, and no such effects are found; indeed, the OH-based abundances show an increase with decreasing Teff, leaving the nature and reality of the cool dwarf [O i]-based O trend uncertain. The mean relative O abundance of the six warmest dwarfs (5075 K TeA 5978 K) is ½O/H¼þ0:14 Æ0:02, and we ﬁnd a mean abundance of ½O/H¼þ0:08 Æ0:02for the giants. Thus, our updated analysis of the [O i] k6300 line does not conﬁrm the Hyades giant-dwarf oxygen discrepancy initially reported by King & Hiltgen, suggesting that the discrepancy was a consequence of analysis-related systematic errors. LTE oxygen abundances from the near-IR, high-excitation O i triplet are also derived for the giants, and the resulting abundances are approximately 0.28 dex higher than those derived from the [O i] line, in agreement with non-LTE predictions. Non-LTE corrections from the literature are applied to the giant triplet abundances; the resulting mean abundance is ½O/H¼þ0:17 Æ0:02, in decent concordance with the giant and dwarf [O i] abundances. Finally, Hyades giant and dwarf O abundances derived from the [O i] k6300 line and high-excitation triplet, as well as dwarf O abundances derived from the near-UV OH k3167 line, are compared, and a mean cluster O abundance of ½O/H¼þ0:12 Æ0:02 is achieved, which represents the best estimate of the Hyades O abundance.
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