EDP Sciences
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Volume 402, Number 1, April IV 2003
Page(s) 343 - 359
Section Formation and evolution of planetary systems
DOI http://dx.doi.org/10.1051/0004-6361:20030217

A&A 402, 343-359 (2003)
DOI: 10.1051/0004-6361:20030217

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

Infrared O I triplet and [O I] lines
Y. Takeda

Komazawa University, Setagaya, Tokyo 154-8525, Japan
(Received 24 May 2002 / Accepted 14 February 2003 )

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 $T_{\rm eff}$, $\log g$, 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 ( $-1 \la [{\rm Fe}/{\rm H}] \la 0$), the existence of a systematic abundance discrepancy was confirmed between O I and [O I] lines in conspicuously metal-poor halo stars ( $-3 \la [{\rm Fe}/{\rm H}] \la -1$) without being removed by our non-LTE corrections, i.e., the former being larger by ~ 0.3 dex at $-3 \la [{\rm Fe}/{\rm H}] \la -2$.

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

Key words: line: formation -- radiative transfer -- Galaxy: abundances -- stars: late-type -- stars: population II

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