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

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 O I IR triplet data. 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 [O I] lambda 6300 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 and coworkers [O/Fe] versus metallicity curve to much lower abundances, with a slope of -0.31 +/- 0.11 (taking into account the error bars in both oxygen abundances and metallicities) in the range -3 < [Fe/H] < -1. The extrapolation of our results to very low metallicities indicates that the first Type II supernovae in the early Galaxy provided oxygen to iron ratios of [O/Fe] greater than or similar to 1. The oxygen abundances of unevolved stars, when compared with values in the literature for giants of similar metallicity, imply that the latter may have suffered a process of oxygen depletion. As a result, unevolved metal-poor stars shah be considered better tracers of the early evolution of oxygen in the Galaxy. The higher [O/Fe] ratios we find in dwarfs has an impact on the age determination of globular clusters, suggesting that current age estimates have to be reduced by about 1-2 Gyr.