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] 6300 Angstrom line, the O I 7774 Angstrom 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 -2.4 < [Fe/H] < -0.5, 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] similar or equal to +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] similar or equal to +0.5 at [Fe/H] = -2.5, but with a tendency of a plateau with [O/Fe] similar or equal to +0.3 for -2.0 < [Fe/H] < -1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] similar or equal to -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.