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O/Fe in metal-poor main sequence and subgiant stars
A study of the O/Fe ratio in metal-poor main sequence and subgiant starsis presented using the \ion{Oi},6300 Å line, the O I 7774 Åtriplet, and a selection of weak Fe Ii lines observed on high-resolutionspectra acquired with the VLT UVES spectrograph. The \ion{Oi line isdetected in the spectra of 18 stars with -2.4 < [Fe/H] < -0.5, andthe triplet is observed for 15 stars with Fe/H ranging from -1.0 to-2.7. The abundance analysis was made first using standard modelatmospheres taking into account non-LTE effects on the triplet: the\ion{Oi} line and the triplet give consistent results with [O/Fe]increasing quasi-linearly with decreasing [Fe/H] reaching [O/Fe] =~ +0.7at [Fe/H] = -2.5. This trend is in reasonable agreement with otherresults for [O/Fe] in metal-poor dwarfs obtained using standardatmospheres and both ultraviolet and infrared OH lines. There is alsobroad agreement with published results for [O/Fe] for giants obtainedusing standard model atmospheres and the \ion{Oi} line, and the OHinfrared lines, but the O I lines give higher [O/Fe] values which may,however, fall into place when non-LTE effects are considered. Whenhydrodynamical model atmospheres representing stellar granulation indwarf and subgiant stars replace standard models, the [O/Fe] from the\ion{Oi} and Fe Ii lines is decreased by an amount which increases withdecreasing [Fe/H]. These 3D effects on [O/Fe] is compounded by theopposite behaviour of the \ion{Oi} (continuous opacity effect) and Fe Iilines (excitation effect). The [O/Fe] vs. [Fe/H] relation remainsquasi-linear extending to [O/Fe] =~ +0.5 at [Fe/H] = -2.5, but with atendency 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 ofthe hydrodynamical models disturbs the broad agreement between theoxygen abundances from the \ion{Oi} , O I, and OH lines, but 3D non-LTEeffects may serve to erase these differences. The [O/Fe] values from the\ion{Oi} line and the hydrodynamical model atmospheres for dwarfs andsubgiant stars are lower than the values for giants using standard modelatmospheres and the \ion{Oi}, and O I lines. Based on observationscollected at the European Southern Observatory, Chile (ESO Nos.65.L-0131, 65.L-0507, and 67.D-0439).
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