THE GIANT BRANCH OF OMEGA-CENTAURI .4. ABUNDANCE PATTERNS BASED ON ECHELLE SPECTRA OF 40 RED GIANTS

Abundances of some 20 elements have been determined for a (biased) sam ple of 40 red giants having M(v) < -1.5 in the chemically inhomogeneou s globular cluster omega Centauri. The results are based on high-resol ution, high signal-to-noise echelle spectra and permit one to examine the roles of primordial enrichment and stellar evolutionary mixing eff ects in the cluster. Our basic conclusions are as follows (1) There is an abundance range -1.8 < [Fe/H] < -0.8, and even more metal rich sta rs may exist in the cluster. (2) For the alpha (Mg, Si, Ca, Ti) and ir on peak (Cr, Ni) elements and Sc and V, [metal/Fe] is flat as a functi on of [Fe/H] and is consistent with primordial enrichment from stars h aving mass greater than 10 M., as has been found for field halo stars. (3) There is a large scatter in the abundances of C, N, and O. The bu lk of the stars have -0.9 < [C/Fe] < -0.3 and [O/Fe] similar to 0.3, a s is found at the red giant branch tip in other ''normal'' (showing no spread in [Fe/H]) clusters of similar abundance, while there also exi sts a group of CN-strong stars having [C/Fe] similar to -0.7 and [O/Fe ] similar to -0.5. Nitrogen appears to be enhanced in all of these car bon-depleted stars. These results are most readily explained in terms of evolutionary mixing effects not predicted by standard stellar evolu tion calculations and are consistent with the earlier suggestions of C ohen & Bell (1986) and Paltoglou & Norris (1989) concerning processing in both the CN and ON cycles in the stars being observed. In contrast , the group of CO-strong stars first identified by Persson et al. (198 0) has [C/Fe] similar to 0.0, [O/Fe] - 0.4, and [N/Fe] similar to 0.4 (or 0.9 if the nitrogen scale of Brown and Wallerstein is correct) and is suggestive of primordial enrichment of carbon and/or nitrogen from intermediate- and possibly low-mass stars, tempered by later stellar evolutionary effects. (4) [Na/Fe] and [Al/Fe] are anticorrelated with [O/Fe], and there is a positive correlation between [Na/Fe] and [Al/Fe ], all of which are most readily explained in terms of evolutionary mi xing effects as first suggested by Denisenkov and Denisenkova (1990). Such an explanation is supported by the similar ([Na/Fe], [O/Fe]) anti correlation reported by Kraft et al. (1993) in the ''normal'' globular clusters. (5) For the heavy neutron-addition elements (in particular Y, Ba, La, and Nd) [heavy metal/Fe] rises as [Fe/H] increases, in shar p contrast with what is found in the ''normal'' clusters, while the re lative abundances as a function of atomic number are suggestive of s-p rocessing. The increase in [heavy metal/Fe] with [Fe/H] appears indepe ndent of the abundance of C, N, O, Na and Al and is most naturally exp lained as a primordial effect. Guided by the predictions of existing ( somewhat uncertain) stellar evolution calculations, we suggest that th is results from primordial enrichment from stars having mass as low as 1-3 M.. (6) If the preceding suggestion is correct, chemical enrichme nt in omega Cen occurred over an extended period, perhaps greater than or equal to 1 Gyr.