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Hyades Oxygen Abundances from the λ6300 [O I] Line: The Giant-Dwarf Oxygen Discrepancy Revisited1,
We present the results of our abundance analysis of Fe, Ni, and O inhigh signal-to-noise ratio, high-resolution Very Large Telescope UVESand McDonald 2dcoudé spectra of nine dwarfs and three giants inthe Hyades open cluster. The difference in Fe abundances derived from FeII and Fe I lines ([Fe II/H]-[Fe I/H]) and Ni I abundances derived frommoderately high-excitation (χ~4.20 eV) lines is found to increasewith decreasing Teff for the dwarfs. Both of these findingsare in concordance with previous results ofoverexcitation/overionization in cool young dwarfs. Oxygen abundancesare derived from the [O I] λ6300 line, with careful attentiongiven to the Ni I blend. The dwarf O abundances are in star-to-staragreement within uncertainties, but the abundances of the three coolestdwarfs (4573 K<=Teff<=4834 K) evince an increase withdecreasing Teff. Possible causes for the apparent trend areconsidered, including the effects of overdissociation of O-containingmolecules. O abundances are derived from the near-UV OH λ3167line in high-quality Keck HIRES spectra, and no such effects are found;indeed, the OH-based abundances show an increase with decreasingTeff, leaving the nature and reality of the cool dwarf [OI]-based O trend uncertain. The mean relative O abundance of the sixwarmest dwarfs (5075 K<=Teff<=5978 K) is[O/H]=+0.14+/-0.02, and we find a mean abundance of [O/H]=+0.08+/-0.02for the giants. Thus, our updated analysis of the [O I] λ6300line does not confirm the Hyades giant-dwarf oxygen discrepancyinitially reported by King & Hiltgen, suggesting that thediscrepancy was a consequence of analysis-related systematic errors. LTEoxygen abundances from the near-IR, high-excitation O I triplet are alsoderived for the giants, and the resulting abundances are approximately0.28 dex higher than those derived from the [O I] line, in agreementwith non-LTE predictions. Non-LTE corrections from the literature areapplied to the giant triplet abundances; the resulting mean abundance is[O/H]=+0.17+/-0.02, in decent concordance with the giant and dwarf [O I]abundances. Finally, Hyades giant and dwarf O abundances derived fromthe [O I] λ6300 line and high-excitation triplet, as well asdwarf O abundances derived from the near-UV OH λ3167 line, arecompared, and a mean cluster O abundance of [O/H]=+0.12+/-0.02 isachieved, which represents the best estimate of the Hyades O abundance.This paper includes data taken with the Harlan J. Smith 2.7 m and theOtto Struve 2.1 m telescopes at the McDonald Observatory of theUniversity of Texas at Austin.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California and theNational Aeronautics and Space Administration. The Observatory was madepossible by the generous financial support of the W. M. Keck Foundation.
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