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Galactic [O/Fe] and [C/Fe] Ratios: The Influence of New Stellar Parameters
The Astronomical Journal
  • Jeremy R King, Clemson University
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The American Astronomical Society
We consider the effects of recent NLTE gravities and Fe abundances on stellar [O/Fe] and [C/Fe] ratios. The NLTE parameters greatly reduce or eliminate the well-known discrepancy between CH- and C I–based C abundances in metal-poor stars and previously seen trends of atomic-based [C/Fe] and [O/Fe] with Teff. With the NLTE parameters, the metal-poor molecular-based [C/Fe] ratio maintains its increase with declining [Fe/H]; this may also be demonstrated by the revised atomic-based ratios. [O/Fe] values derived from OH and O I features are considerably reduced and typically show improved agreement but are 0.1–0.2 dex larger than those exhibited by the Lick-Texas syndicate's recent [O I]–based giant determinations. The revised [O/Fe] ratios still show an increase down to at least [Fe/H] ~ -2; we suggest that recent field giant data show an increase with similar slope. Even adopting uniform NLTE parameters, study-to-study abundance differences can be significant; moreover, different NLTE studies yield differing gravities and Fe abundances even after taking Teff differences into account. Comparison of metal-poor giant gravities and cluster abundances with isochrones, trigonometric gravities, and near-turnoff cluster abundances yields conflicting indications about whether the evolved gravities might be underestimated as suggested for metal-poor dwarfs. Regardless, we argue that even extreme gravity revisions do not affect the [O/Fe]-[Fe/H] relation derived from the extant results. Combining what we believe the most reliable giant and dwarf data considered here, we find [O/Fe] = -0.184(±0.022) × [Fe/H] + 0.019 with an rms scatter of only 0.13 dex; there is no indication of a break or slope change at intermediate [Fe/H]. The gentle slope is in very reasonable agreement with some chemical evolution models employing yields with small mass and metallicity dependences. Finally, two notes are made concerning Na abundance-spatial position and element-to-element correlations in M13 giants.
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