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

Open Access
Publisher Website
Google Scholar
Abstract
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.