THE CONSTANCY OF [ALPHA FE] IN GLOBULAR-CLUSTERS OF DIFFERING [FE/H] AND AGE/

We review the literature to derive estimates for the abundances of the four ''alpha'' elements' abundances relative to iron, [O/Fe], [Si/Fe] , [Ca/Fe], and [Ti/Fe], as a function of [Fe/H] and age, in the globul ar clusters. Magnesium is understudied and is depleted in some metal-p oor cluster giants, and hence is not useful for study. While [O/Fe] va lues within clusters vary, due to mixing of ON cycle material into the ir photospheres, consideration of only those stars with low sodium abu ndances, [Na/Fe]less than or equal to 0.00, shows that the unmixed sta rs in clusters have uniformly high [O/Fe] values, from [Fe/H]=-2.24 to -0.58. The ''mean alpha'' values from two of the other three elements , silicon and titanium, also do not appear to vary. [Ca/Fe] does appea r to drop as [Fe/H] increases or age decreases, but this may indicate a problem with the use of neutral calcium lines. Thus, there does not appear to be any sign of Type Ia supernovae contributions to the globu lar clusters, despite wide ranges in [Fe/H] and age. If the three glob ular-cluster classes, ''old halo,'' ''young halo,'' and ''disk,'' are related to one another, if our relative cluster ages are correct, and if the Type Ia supernova model is the correct explanation of the decli ning [alpha/Fe] ratios observed among field stars, then its time scale must be much longer than 10(9) yr. If the time scale is that short, t hen at a minimum, it is clear that the old halo and disk globular clus ters do not share a common chemical history, despite both showing prog rade Galactic rotation. In that case, one of the classes presumably fo rmed farther from the Galaxy and was later accreted by it.