Oxygen abundances in metal-poor subgiant stars from the O I triplet

This paper examines the oxygen abundance of 24 main-sequence turn-off-and subgiant stars through the permitted O I triplet near lambda 7774 Angstrom. In agreement with prior investigators, we find that the O abundance from the triplet is about 0.53 dex higher than is typically found from other means such as the [O I] lambda 6300 Angstrom Line or the UV and LR bands from OH. Our average [O/Fe] is +0.79+/-0.29 (1 sigma), with the uncertainties in the individual methods being dominated by systematic errors in our incomplete understanding of the formation of these high-excitation lines under non-LTE conditions. Several methods of resolving the ongoing discrepancy are discussed in detail. We find that the problem is not fixed by simply changing the model. atmosphere parameters, and that the best methods seem to require a detailed non-LTE analysis based on both the structure of the oxygen atom and the source function within and above the photosphere. The trend of increasing O with decreasing Fe is briefly explored and is found to be qualitatively consistent with other works.