ABUNDANCE DIFFERENCES AMONG GLOBULAR-CLUSTER GIANTS - PRIMORDIAL VERSUS EVOLUTIONARY SCENARIOS

Contrary to historical expectation, stars within a given globular clus ter often exhibit wide variations in the abundance of C, N, and O as w ell as certain light metals, particularly Na and Al. Owing to flux lim itations, studies have been confined to evolved stars, especially gian ts, but in few instances variations have been detected among main-sequ ence stars. Variations in the Fe-peak elements exceeding approximately 0.1 dex are firmly established in the case of omega Centauri, the mos t massive cluster, and are strongly suspected in the case of M22, but in no other cluster. Among field halo giants of comparable Fe-peak met allicity, variations in the C, N, 0 group appear to be much less prono unced than in globular-cluster giants. Among giants, the variations ar e of two kinds: (1) those related on the average to evolutionary state , and (2) variations among stars in the same apparent evolutionary sta te. In addition, clusters having the same Fe-peak abundances often con tain stars with very different ''signatures'' of oxygen and CN-band st rengths. The abundances of C and N are often anticorrelated, and in th e limited number of cases in which both have been measured, O and N ab undances have also often proved to be anticorrelated (Pilachowski 1988 ; Sneden et al. 1991; Brown et al. 1991; Kraft et al. 1992). Following pioneering work by Cohen (1978) and Peterson (1980), strong evidence has recently emerged for the existence of a significant global anticor relation between 0 and Na abundances (Drake et al. 1992, Kraft et al. 1993). The observations are discussed in terms of contrasting hypothes es: evolutionary versus primordial. In the former, the variations are attributed to the dredgeup of material that has been processed through the CNO cycle in the globular-cluster stars themselves. In the latter , the variations are attributed to primordial chemical inhomogeneities in the material out of which the cluster stars were formed, the compo sition of these ''clumps'' having been determined by nuclear processin g in a prior generation of more massive stars. Observational evidence supporting each of these scenarios is cited. Recent studies of stellar rotation among horizontal branch stars in certain clusters (Peterson et al. 1994) as well as new calculations of Na-23 and Al-27 production in the CNO processing regions of evolving low-mass giants (Langer et al. 1993) lend fresh support to the evolutionary hypothesis. However, such calculations do not explain the variation of C and N abundances f ound among cluster main-sequence stars (Suntzeff 1989; Briley et al. 1 991) which therefore seem explicable only on the basis of a primordial scenario. Among mildly metal-poor giants, i.e., those in the range fr om solar metallicity to [Fe/H]approximately -1, recent observational e vidence suggesting the existence of a substructure in the [el/Fe] rati os of the heavier alpha elements, e.g., Si, Mg, Ca, and Ti, is discuss ed. The possible influence of this effect on the interpretation of the integrated spectra of extragalactic globular clusters and E galaxies is noted.