OXYGEN ABUNDANCES IN HALO STARS

The oxygen abundance has been determined for a sample of metal-poor G dwarfs (-1.2 < [Fe/H] < -2.7) in two independent ways; by analysis of OH lines between 3080-3200 angstrom and the permitted high-excitation far-red O I triplet (7771-7775 angstrom, chi = 9.1 eV). The oxygen abundances determined from the low-excitation OH lines are up to 0.55 dex lower than those measured from the high-excitation O I lines. The abundances for the far-red O I triplet lines agree with those rederived from Abia and Rebolo and the abundances from the OH lines in dwarfs and giants are in agreement with the rederived O abundances of Barbuy and others from the forbidden resonance O I line (6300 angstrom, chi = 0.0 eV). Because the chi = 0-1.7 eV OH lines are formed in the same layers as the majority of Fe, Ti and other neutral metal lines used for abundance analyses we believe that the OH lines and the forbidden O I line yield the true oxygen abundances relative to the metals. The most likely explanation for the systematic overabundances derived from high-excitation lines is that, due to convection, metal-poor stars are much hotter in the deep layers where these lines are formed than are the model atmospheres used in the analyses. Discounting the oxygen abundances from the O I triplet, the [O/Fe] ratios in metal-poor stars now resemble those of other alpha-elements. The [O/Fe] versus [Fe/H] curve differs from that commonly accepted and [O/Fe] could be 0.25 at [Fe/H] = -1, about half the value previously adopted with the flattening out in overabundance occurring not near [Fe/H] approximately -1 but beyond approximately -1.7. A plateau value for [O/Fe] of 0.5 to 0.6 is indicated for stars with metallicities below [Fe/H] = -2.0. [O/C] is zero for the three most metal-deficient dwarfs examined.