No 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 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 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 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 et al. 1994) as well as new calculations of 23 Na and 27 Al production in the CNO processing regions of evolving low-mass giants ) 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 therefore seem explicable 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 [e//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.

We present a chemical composition analysis of three dozen giant stars in the nearby "CN-bimodal" mildly metal-poor (<[Fe/H]> = -1.18) globular cluster M4. The analysis combined traditional spectroscopic abundance methods with modifications to the line-depth ratio technique pioneered by Gray (1994). Silicon and aluminum are found to be primordially overabundant by factors exceeding the mild overabundances usually seen in α-and light odd elements among halo field and globular cluster giants of comparable metallicity. In addition, barium is found to be overabundant by a factor of about four. Superimposed on the primordial abundance distribution in M4, there is evidence for the existence of proton-capture synthesis of C, O, Ne, and Mg.

When using Fe as a surrogate for "metallicity", the metallicity is best represented by the dominant species of Fe. Accordingly, we have derived a new globular cluster metallicity scale based on the equivalent widths of Fe II lines measured from high resolution spectra of giant stars. The scale is primarily based on the results of analyses by the Lick-Texas group of 149 stars in 11 clusters, supplemented by other high resolution studies in five additional clusters. We also derive ab initio the true distance moduli for M3, M5, M13, M15, and M92 as a means of setting stellar surface gravities. We find that [Fe/H] II is correlated linearly with W ′ , the reduced strength of the near-infrared Ca II triplet defined by Rutledge et al (1997), although the correlation coefficients depend on the stellar atmosphere model employed. In addition to the 66 globular cluster metallicity estimates presented in a recent PASP review, we present here an additional 39 globular cluster metallicity estimates based on transformations from Q39, the photometric index defined by Zinn (1980). Kraft & Ivans1993) model atmospheres with overshooting, and a uniform colour-effective temperature ("T eff ") relationship (Gratton et al 1996) for 160 stars in 24 globular clusters. The difference in the metallicity scales, in the sense of ∆[Fe/H] = CG97 -ZW84, is -0.1 dex for metal-rich clusters and ∼ +0.2 dex for metal-poor clusters.

We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M 22. High resolution spectra for this study were obtained at five observatories, and analyzed in a uniform manner. We have determined abundances of representative light proton-capture, α, Fe-peak and neutron-capture element groups. Our aim is to better understand the peculiar chemical enrichment history of this cluster, in which two stellar groups are characterized by a different content in iron, neutron capture elements Y, Zr and Ba, and α element Ca