Oxygen Abundances in Very Metal-Poor Stars

Abstract

Oxygen abundances have been derived in a sample of very metal-poor stars from observations of the O i triplet at λλ7771–7775Å and OH lines in the near ultraviolet. A detailed NLTE analysis of iron lines has been carried out for one of the observed stars, BD +23° 3130, providing consistent values of effective temperature, surface gravity and metallicity, that are in good agreement with independent estimates from other methods. These parameters reduce the discrepancies claimed between the oxygen abundances determined from OH and [O i] λ6300Å lines, and give a similar abundance from the O i triplet for BD +23° 3130. This consistency (found using 1D hydrostatic model atmospheres) strongly constrains the effects predicted by 3D hydrodynamical models on these three indicators. The oxygen abundances derived for this new sample confirm previous findings for a progressive linear increase in the oxygen-to-iron ratio with a slope −0.33 ± 0.02 for stars with metallicties in the range 0 >[Fe/H]> −3, and [0/Fe] values as high as ~ 1.1 for stars with [Fe/H]< −2.5.