Oxygen Abundances in Metal Poor Subgiant Stars from the O I Triplet

Abstract

This paper examines the oxygen abundance of 24 main-sequence turn-off and subgiant stars through the permitted O I triplet near λ7774 {Å}. 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] λ6300 {Å} line or the UV and IR bands from OH. Our average [O/Fe] is +0.79 ± 0.29 (1 σ), 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 on-going 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.