Oxygen-Line Formation in Population-II A-Type Stars

Abstract

The non-LTE formation of neutral oxygen lines in the visible (λ 6156–8) and near-IR (λ 7771–5) region was studied, and extensive grids of theoretical non-LTE equivalent widths and abundance corrections were computed in the parameter range of −2 ≤ [Fe/H] ≤ 0, 7000 K ≤Teff ≤ 14000 K, and 3.0 ≤ log g ≤ 4.0, with the intention of applying them to A-type stars, especially Population-II stars such as horizontal-branch objects. It was found that the inclusion of UV Lyman lines (such as Lyman β overlapping with a strong OI resonance transition) in the background opacity may be important, which tends to suppress the non-LTE effect, especially for the metal-deficient case. Nevertheless, the resulting non-LTE correction for OI 7771–5 is still of considerable amount (≳ 0.5–1.0 dex), even in the weak-line cases of low abundance, and should inevitably be taken into account when determining oxygen abundance from this near-IR triplet.