Oxygen line formation in late-F through early-K disk/halo stars

Abstract

In order to investigate the formation of O i 7771–5 and [O i] 6300/6363 lines, extensive non-LTE calculations for neutral atomic oxygen were carried out for wide ranges of model atmosphere parameters, which are applicable to early-K through late-F halo/disk stars of various evolutionary stages. The formation of the triplet O i lines was found to be well described by the classical two-level-atom scattering model, and the non-LTE correction is practically determined by the parameters of the line-transition itself without any significant relevance to the details of the oxygen atomic model. This simplifies the problem in the sense that the non-LTE abundance correction is essentially determined only by the line-strength (), if the atmospheric parameters of Teff, , and ξ are given, without any explicit dependence of the metallicity; thus allowing a useful analytical formula with tabulated numerical coefficients. On the other hand, our calculations lead to the robust conclusion that LTE is totally valid for the forbidden [O i] lines. An extensive reanalysis of published equivalent-width data of O i 7771–5 and [O i] 6300/6363 taken from various literature resulted in the conclusion that, while a reasonable consistency of O i and [O i] abundances was observed for disk stars (), the existence of a systematic abundance discrepancy was confirmed between O i and [O i] lines in conspicuously metal-poor halo stars () without being removed by our non-LTE corrections, i.e., the former being larger by ~0.3 dex at . An inspection of the parameter-dependence of this discordance indicates that the extent of the discrepancy tends to be comparatively lessened for higher stars, suggesting the preference of dwarf (or subgiant) stars for studying the oxygen abundances of metal-poor stars.