Oxygen line formation in late-F through early-K disk/halo stars. Infrared O I triplet and [O I] lines

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 (W_lambda ), if the atmospheric parameters of T_eff, log g, and xi 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 (-1 <~ [Fe/H] <~ 0), the existence of a systematic abundance discrepancy was confirmed between O I and [O I] lines in conspicuously metal-poor halo stars (-3 <~ [Fe/H] <~ -1) without being removed by our non-LTE corrections, i.e., the former being larger by ~ 0.3 dex at -3 <~ [Fe/H] <~ -2.

An inspection of the parameter-dependence of this discordance indicates that the extent of the discrepancy tends to be comparatively lessened for higher T_eff/log g stars, suggesting the preference of dwarf (or subgiant) stars for studying the oxygen abundances of metal-poor stars.

Tables 2, 5, and 7 are only available in electronic form, at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/343 and Table \ref{tab3} is only available in electronic form at http://www.edpsciences.org