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

Abstract

A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is presented using the \ion{O{i}},6300 Å line, the \ion{O}{i} 7774 Å triplet, and a selection of weak \ion{Fe}{ii} lines observed on high-resolution spectra acquired with the VLT UVES spectrograph. The \ion{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 \ion{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 \ion{O{i}} line, and the OH infrared lines, but the \ion{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 \ion{O{i}} and \ion{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 \ion{O{i}} (continuous opacity effect) and \ion{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 \ion{O{i}} , \ion{O}{i}, and OH lines, but 3D non-LTE effects may serve to erase these differences. The [O/Fe] values from the \ion{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 \ion{O{i}}, and \ion{O}{i} lines. Based on observations collected at the European Southern Observatory, Chile (ESO Nos. 65.L-0131, 65.L-0507, and 67.D-0439). Show more