ABUNDANCE DIFFERENCES AMONG GLOBULAR CLUSTER GIANTS: PRIMORDIAL VS. EVOLUTIONARY SCENARIOS

Abstract

ABSTRACT Contrary to historical expectation, stars within a given globular cluster often exhibit wide variations in the abundance of C, N, and O as well as certain light metals, particularly Na and Al. Owing to flux limitations, studies have been confined to evolved stars, especially to giants, but in few instances variations have been detected among main sequence stars. Variations in the Fe-peak elements exceeding ~0.1 dex are firmly established in the case of omega Centauri, the most massive cluster, and are strongly suspected in the case of M22, but in no other cluster. Among field halo giants of comparable Fe-peak metallicity, variations in the C, N, O group appear to be much less pronounced than in globular cluster giants. Among giants, the variations are of two kinds: (1) those related on the average to evolutionary state, and (2) variations among stars in the same apparent evolutionary state. In addition, clusters having the same Fe-peak abundances often contain stars with very different "signatures" of oxygen and CN-band strengths. The abundances of C and N are often anticorrelated, and in the limited number of cases in which both have been measured, O and N abundances 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 anticorrelation between O and Na abundances (Drake et al. 1992, Kraft et al. 1993). The observations are discussed in terms of contrasting hypotheses: evolutionary vs. 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 the primordial chemical inhomogeneities in the material out of which the cluster stars were formed, the composition of these "clumps" having been determined by nuclear processing 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 1993) as well as new calculations of 23Na and 27Al production in te 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 found among cluster main sequence stars (Suntzeff 1989, Briley et al. 1991) which seem explicable therefore only on the basis of a primordial scenario. Among mildly metal-poor giants, i.e., those in the range from solar metallicity to [Fe/H] ~-1, recent observational evidence suggesting the existence of a substructure in the [el/Fe]-ratios of the heavier alpha elements, e.g., Si, Mg, Ca and Ti, is discussed. The possible influence of this effect on the interpretation of the integrated spectra of extragalactic globular clusters and E galaxies is noted.