Oxygen Line Formation in Late-F through Early-K Disk/Halo Stars: Infrared O I Triplet and [O I] Lines

TL;DR

This study performs non-LTE calculations for oxygen lines in late-F to early-K stars, providing simplified correction formulas and confirming LTE validity for forbidden lines, while revealing abundance discrepancies in metal-poor halo stars.

Contribution

It offers a simplified analytical approach for non-LTE corrections of O I triplet lines and confirms LTE validity for [O I] lines, with a reanalysis of observational data highlighting abundance discrepancies in metal-poor stars.

Findings

Non-LTE correction depends mainly on line strength, not metallicity.

LTE is valid for forbidden [O I] lines.

Systematic abundance discrepancy in metal-poor halo stars.

Abstract

In order to investigate the formation of O {\sc i} 7771--5 and [O {\sc 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 {\sc 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 {\sc i}] lines. An extensive reanalysis of published equivalent-width data of O {\sc i} 7771--5 and [O {\sc i}] 6300/6363 taken from various literature resulted in the conclusion that, while a reasonable consistency of O {\sc i} and [O {\sc 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 {\sc i} and [O {\sc 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 $\sim 0.3$ dex at $-3 \la [{\rm Fe}/{\rm H}] \la -2$.