Carbon and oxygen abundances of field RR Lyrae stars. II - Oxygen abundances

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Carbon, Horizontal Branch Stars, Oxygen, Stellar Composition, Abundance, Iron, Spectral Line Width

Scientific paper

From non-LTE analysis of the neutral oxygen triplet (7771-7775 Å) absorption-line strengths on KPNO 4 m echelle plates, we have determined oxygen abundances and oxygen-to-iron ratios for a large number of field RR Lyrae stars covering the range in iron abundance +0.1> [Fe/H]> - 2.2. We have found that (1) RR Lyrae stars at the metal-poor end of the range have [O/Fe] values near zero. This is somewhat surprising since previous studies of metal-poor field stars have found oxygen to be enhanced relative to iron. (2) RR Lyrae stars at the metal-rich end of the range have [O/Fe] values between + 0.25 and + 0.95. Again, this result is in disagreement with expectations based on studies of other field stars in the disk. (3) The abundance sum C + N + O, or its possible range, has been determined. For most stars, including those at the metal-rich end of the range, the ratio (C + N + O)/Fe is high relative to the solar value. Possible explanations for these results are discussed.

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