Evolution of O Abundance Relative to Fe

We present a three-component mixing model for the evolution of O abundancerelative to Fe, taking into account the contributions of the first very massive(> 100 solar masses) stars formed from Big Bang debris. We show that theobservations of O and Fe abundances in metal-poor stars in the Galaxy byIsraelian et al. and Boesgaard et al. can be well represented bothqualitatively and quantitatively by this model. Under the assumption of aninitial Fe ([Fe/H] = -3) and O inventory due to the prompt production by thefirst very massive stars, the data at -3 < [Fe/H] < -1 are interpreted toresult from the addition of O and Fe only from type II supernovae (SNII) to theprompt inventory. At [Fe/H] = -1, SNII still contribute O while both SNII andtype Ia supernovae contribute Fe. During this later stage, (O/Fe) sharply dropsoff to an asymptotic value of 0.8(O/Fe)_sun. The value of (O/Fe) for the promptinventory at [Fe/H] = -3 is found to be (O/Fe) = 20(O/Fe)_sun. This resultsuggests that protogalaxies with low ``metallicities'' should exhibit highvalues of (O/Fe). The C/O ratio produced by the first very massive stars isexpected to be much less than 1 so that all the C should be tied up as CO andthat C dust and hydrocarbon compounds should be quite rare at epochscorresponding to [Fe/H] < -3.Comment: 25 pages, 8 postscript figures, to appear in Ap