Context :

Aims : A detailed study is presented , including estimates of the impact on elemental abundance analysis , of the non–Local Thermodynamic Equilibrium ( non–LTE ) formation of the high-excitation neutral oxygen 777 nm triplet in model atmospheres representative of stars with spectral types F to K .

Methods : We have applied the statistical equilibrium code MULTI to a number of plane–parallel MARCS atmospheric models covering late-type stars ( 4500 \leq T _ { eff } \leq 6500 K , 2 \leq \mbox { log $g$ } \leq 5 [ cgs ] , and -3.5 \leq \mbox { [ Fe / H ] } \leq 0 ) .
The atomic model employed includes , in particular , recent quantum-mechanical electron collision data .

Results : 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 solar metallicity , the non–LTE effect , mainly attributed in previous studies to photon losses in the triplet itself , is also driven by an additional significant contribution from line opacity .
At low metallicity , the very pronounced departures from LTE are due to overpopulation of the lower level ( 3s ^ { 5 } S ^ { o } ) of the transition .
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 | \Delta \log \epsilon _ { O } | \sim 0.9 dex and \sim 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 | \Delta \log \epsilon _ { O } | \sim 0.5 dex and \sim 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 .

Conclusions :