O / Fe in metal - poor main sequence and subgiant stars ⋆

A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is presented using the [O i] 6300Å line, the O i 7774Å triplet, and a selection of weak Fe ii lines observed on high-resolution spectra acquired with the VLT UVES spectrograph. The [O i] line is detected in the spectra of 18 stars with −0.5 < [Fe/H] − 2.4, and the triplet is observed for 15 stars with [Fe/H] ranging from −1.0 to −2.7. The abundance analysis was made first using standard model atmospheres taking into account non-LTE effects on the triplet: the [O i] line and the triplet give consistent results with [O/Fe] increasing quasi-linearly with decreasing [Fe/H] reaching [O/Fe] ≃ +0.7 at [Fe/H] = −2.5. This trend is in reasonable agreement with other results for [O/Fe] in metal-poor dwarfs obtained using standard atmospheres and both ultraviolet and infrared OH lines. There is also broad agreement with published results for [O/Fe] for giants obtained using standard model atmospheres and the [O i] line, and the OH infrared lines, but the O i lines give higher [O/Fe] values which may, however, fall into place when non-LTE effects are considered. When hydrodynamical model atmospheres representing stellar granulation in dwarf and subgiant stars replace standard models, the [O/Fe] from the [O i] and Fe ii lines is decreased by an amount which increases with decreasing [Fe/H]. These 3D effects on [O/Fe] is compounded by the opposite behaviour of the [O i] (continuous opacity effect) and Fe ii lines (excitation effect). The [O/Fe] vs [Fe/H] relation remains quasi-linear extending to [O/Fe] ≃ +0.5 at [Fe/H] = −2.5, but with a tendency of a plateau with [O/Fe] ≃ +0.3 for −2.0 < [Fe/H] < −1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] ≃ −1.0. Use of the hydrodynamical models disturbs the broad agreement between the oxygen abundances from the [O i], O i and OH lines, but 3D non-LTE effects may serve to erase these differences. The [O/Fe] values from the [O i] line and the hydrodynamical model atmospheres for dwarfs and subgiant stars are lower than the values for giants using standard model atmospheres and the [O i] and O i lines.