Oxygen abundances in unevolved metal-poor stars from near-UV OH lines

Abstract

We have performed a detailed oxygen abundance analysis of 23 metal-poor (-3.0<[Fe/H]<-0.3) unevolved halo stars and one giant through the OH bands in the near UV, using high-resolution echelle spectra. Oxygen is found to be overabundant with respect to iron in these stars, with the [O/Fe] ratio increasing from 0.6 to 1 between [Fe/H]=-1.5 and -3.0. The behavior of the oxygen overabundance with respect to [Fe/H] is similar to that seen in previous works based on OI IR triplet data (Abia and Rebolo 1989; Tomkin et al. 1992; Cavallo, Pilachowski, and Rebolo 1997). Contrary to the previously accepted picture, our oxygen abundances, derived from low-excitation OH lines, agree well with those derived from high-excitation lines of the triplet. For nine stars in common with Tomkin et al. we obtain a mean difference of 0.00+/-0.11 dex with respect to the abundances determined from the triplet using the same stellar parameters and model photospheres. For four stars in our sample we have found measurements of the [OI] 6300 A line in the literature, from which we derive oxygen abundances consistent (average difference 0.09 dex) with those based on OH lines, showing that the long standing controversy between oxygen abundances from forbidden and permitted lines in metal-poor unevolved stars can be resolved. Our new oxygen abundances show a smooth extension of the Edvardsson et al.'s (1993) [O/Fe] versus metallicity curve to much lower abundances, with a slope -0.31+/- 0.11 (taking into account the error bars in both oxygen abundances and metallicities) in the range -3<[Fe/H]<-1.