1999AJ....117..492B

The determination of the abundance of oxygen (O) is important in our understanding of mass-spectrum of previous generations of stars, the evolution of the Galaxy, stellar evolution, and the age-metallicity relation. We have measured O in 24 unevolved stars with Keck HIRES observations of the OH lines in the ultraviolet spectral region at a spectral resolution of ∼45,000. The spectra have high signal-to-noise ratios, typically 60-110, and high dispersion, 0.022 Å per pixel. Very special care has been taken in determining the stellar parameters in a consistent way and we have done this for two different, plausible temperature scales. The O abundance from OH has been computed by spectrum synthesis techniques for all 24 stars plus the Sun for which we have a Keck spectrum of the daytime sky. In addition, we determined O abundances from the O I triplet with our stellar parameters and the published equivalent widths of the three O I lines from six sources. The comparison of data analyzed with the same, consistently determined, parameter sets show generally excellent agreement in the O abundances; differences in the origin of the models (not the parameters) may result in abundance differences of 0.07 to 0.11 dex. We show that the O abundances from OH and from O I are reliable and independent and average the two for the adopted O. This averaging has the great benefit of neutralizing uncertainties in the parameters since OH and O I strengths depend on effective temperature and gravity in opposite directions. For these cool, unevolved stars we find that O is enhanced relative to Fe with a completely linear relation between [O/H] and [Fe/H] over 3 orders of magnitude with very little scatter; taking the errors into account in determining the fits, we find [O/H] = +0.66 (±0.02) [Fe/H] + 0.05 (±0.04). The O abundances from 76 disk stars of Edvardsson et al. have a measured slope of 0.66 (identical to our halo dwarf stars) and fit this relationship smoothly. The relation between [O/Fe] and [Fe/H] is robustly linear and shows no sign of a break at metallicities between -1.0 and -2.0, as has been discussed by others. At low metallicities, [Fe/H] < -3.0, [O/Fe] > +1.0. The fit to this relationship (taking the errors into account) is [O/Fe] = -0.35 (±0.03) [Fe/H] + 0.03 (±0.05). The enrichment of O is probably still from massive stars and Type II supernovae; however, the absence of a break in [O/Fe] versus [Fe/H] runs counter to traditional galactic evolution models, and the interplay of Type II and Type Ia supernovae in the production of O and Fe should be reexamined. It appears that either Fe or O can be used as a chronometer in studies of galactic evolution.