Non-LTE Analysis of the Oxygen λλ 7771–5 Triplet in Mid-G through Early-K Gaints

Abstract

An extensive spectral-synthesis analysis in the neighborhood of the OI 7771–5 lines (including also the lines of FeI, NdII, and CN) was carried out for thirty-two giant stars at the spectral type of mid-G through early-K, and their photospheric oxygen abundances were derived from these triplet lines based on detailed statistical-equilibrium calculations, aiming to assess the importance of the non-LTE effect quantitatively as well as to examine the reliability of the abundances derived from this near-IR triplet compared to those obtained from the forbidden line widely invoked. It turned out that the expected (negative) non-LTE correction ranges from 0.1 to 0.2 dex, the extent of which is somewhat smaller than has been reported by Eriksson and Toft (1979, AAA 25.114.007). The resulting non-LTE abundances were found to agree in most cases with those determined from the published equivalent widths of the forbidden O I 6300 line with no serious systematic descrepancy, though some showing appreciable scatters. The possible sources of uncertainties in oxygen-abundance determination are further examined, and the relevant characteristics of the resulting abundances, such as the [O/Fe] ratio in G–K giants, are discussed.