Oxygen vs. age and metallicity in halo field stars

Oxygen abundances as a function of age and metallicity have been determined for a sample of 37 metal-poor held stars near the main sequence turn-off. We have measured oxygen and iron abundances using the infrared oxygen triplet and numerous Fe lines, and ages have previously been determined for these stars by Schuster & Nissen from Stromgren photometry and theoretical isochrones. The transition to lower [O/Fe] at [Fe/H] approximate to -1 is much better defined with the addition of these new data, and there may be a somewhat larger scatter in [O/Fe] on the metal-rich side of the transition. Two stars are found to be oxygen-poor, and join a small number of other dwarf and subgiant halo stars known to be poor in alpha elements. A correlation of [O/Fe] and age, expected from the later contribution of Type Ia supernovae, is not found. If the age range is real, this suggests that the stars of different ages formed in chemically isolated regions of the halo, or that the timescale for Type Ia supernovae is significantly longer than 1 Gyr.