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

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.

💡 Research Summary

This paper presents a comprehensive re‑examination of the non‑Local Thermodynamic Equilibrium (non‑LTE) formation of the high‑excitation neutral oxygen (O I) triplet at 777 nm in late‑type stellar atmospheres, using MARCS 1‑D model atmospheres representative of spectral types F through K. The authors employed the statistical‑equilibrium code MULTI, incorporating a newly constructed atomic model that includes the latest quantum‑mechanical electron‑collision cross‑sections, particularly those governing the intersystem coupling between the triplet and quintet systems.

The study confirms that the O I 777 nm lines are significantly affected by non‑LTE effects across the parameter space examined. In all cases the lines are stronger in LTE than in reality, leading to negative abundance corrections (ΔNLTE < 0). The magnitude of these corrections grows dramatically toward lower metallicities. For metal‑poor turn‑off stars with