Oxygen in Unevolved Metal-Poor Stars from Keck Ultraviolet HIRES Spectra

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 D45,000. The spectra have high signal-to-noise ratios, typically 60¨110, and high dispersion, 0.022 per pixel. Very special care has been Ae taken in determining the stellar parameters in a consistent way and we have done this for two diUerent, plausible temperature scales. The O abundance from OH has been computed by spectrum synthesis tech- niques 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 equiva- lent 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; diUer- ences in the origin of the models (not the parameters) may result in abundance diUerences 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 bene—t of neutralizing uncertainties in the parameters since OH and O I strengths depend on eUective temperature and gravity in opposite direc- tions. For these cool, unevolved stars we —nd 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 —ts, we —nd (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 —t 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 —t 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 prob- ably 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.