Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity

TL;DR

This study revisits the formation of the neutral oxygen 777 nm triplet in stellar atmospheres, revealing significant non-LTE effects at low metallicity due to new collisional data, impacting oxygen abundance estimates.

Contribution

It introduces updated quantum-mechanical electron collision data into non-LTE modeling of oxygen lines, highlighting large metallicity-dependent effects previously underestimated.

Findings

Non-LTE effects cause large negative abundance corrections at low metallicity.

Including recent collision data reduces but does not eliminate LTE overestimates.

Non-LTE effects imply lower [O/Fe] ratios in metal-poor halo stars.

Abstract

We present a detailed study of the non-Local Thermodynamic Equilibrium (non-LTE) formation of the high-excitation neutral oxygen 777 nm triplet in MARCS model atmospheres representative of late-type stars with spectral types F to K. We carried out the calculations using the statistical equilibrium code MULTI, including estimates of the impact on elemental abundance analysis. The atomic model employed includes, in particular, recent quantum-mechanical electron collision data. We confirm that the O I triplet lines form under non-LTE conditions in late-type stars, suffering negative abundance corrections with respect to LTE. At low metallicity, large line opacity stems from triplet-quintet intersystem electron collisions, a form of coupling previously not considered or seriously underestimated. The non-LTE effects become generally severe for models (both giants and dwarfs) with higher T_eff. Interestingly, in metal-poor turn-off stars, the negative non-LTE abundance corrections tend to rapidly become more severe towards lower metallicity. When neglecting H collisions, they amount to as much as ~ 0.9 dex and ~ 1.2 dex, respectively at [Fe/H]=-3 and [Fe/H]=-3.5. Even when such collisions are included, the LTE abundance remains a serious overestimate, correspondingly by ~ 0.5 dex and ~ 0.9 dex at such low metallicities. Although the poorly known inelastic hydrogen collisions thus remain an important uncertainty, the large metallicity-dependent non-LTE effects seem to point to a resulting "low" (compared to LTE) [O/Fe] in metal-poor halo stars.