The Giant Branch of omega Centauri. IV. Abundance Patterns Based on Echelle Spectra of 40 Red Giants

Abstract

Abundances of some 20 elements have been determined for a (biased)sample of 40 red giants having M_{v} {\lt} -1.5 in the chemicallyinhomogeneous globular cluster {ω} Centauri. The results are basedon high-resolution, high signal-to-noise echelle spectra and permit oneto examine the roles of primordial enrichment and stellar evolutionarymixing effects in the cluster. Our basic conclusions are as follows (1)There is an abundance range -1.8 {\lt} [Fe/H] {\lt} -0.8, and even moremetal rich stars may exist in the cluster. (2) For the {α} (Mg, Si,Ca, Ti) and iron peak (Cr, Ni) elements and Sc and V, [metal/Fe] is flatas a function of [Fe/H] and is consistent with primordial enrichmentfrom stars having mass greater than 10 M_{sun}, as has beenfound for field halo stars. (3) There is a large scatter in theabundances of C, N, and 0. The bulk of the stars have -0.9 {\lt} [C/Fe]{\lt} -0.3 and [O/Fe] {\tilde} 0.3, as is found at the red giant branchtip in other ''normal'' (showing no spread in [Fe/H]) clusters of similarabundance, while there also exists a group of CN-strong stars having[C/Fe] {\tilde} -0.7 and [O/Fe] {\tilde} -0.5. Nitrogen appears to beenhanced in all of these carbon-depleted stars. These results are mostreadily explained in terms of evolutionary mixing effects not predictedby standard stellar evolution calculations and are consistent with theearlier suggestions of Cohen {\amp} Bell (1986) and Paltoglou {\amp}Norris (1989) concerning processing in both the CN and ON cycles in thestars being observed. In contrast, the group of CO-strong stars firstidentified by Persson et al. (1980) has [C/Fe] {\tilde} 0.0, [O/Fe]{\tilde} 0.4, and [N/Fe] {\tilde} 0.4 (or 0.9 if the nitrogen scale ofBrown and Wallerstein is correct) and is suggestive of primordialenrichment of carbon and/or nitrogen from intermediate- and possiblylow-mass stars, tempered by later stellar evolutionary effects. (4)[Na/Fe] and [Al/Fe] are anticorrelated with [O/Fe], and there is apositive correlation between [Na/Fe] and [Al/Fe], all of which are mostreadily explained in terms of evolutionary mixing effects as firstsuggested by Denisenkov {\amp} Denisenkova (1990). Such an explanation issupported by the similar ([Na/Fe], [O/Fe]) anticorrelation reported byKraft et al. (1993) in the ''normal'' globular clusters. (5) For theheavy neutron-addition elements (in particular Y, Ba, La, and Nd) [heavymetal/Fe] rises as [Fe/H] increases, in sharp contrast with what isfound in the ''normal'' clusters, while the relative abundances as afunction of atomic number are suggestive of s-processing. The increasein [heavy metal/Fe] with [Fe/H] appears independent of the abundance ofC, N, O, Na and Al and is most naturally explained as a primordialeffect. Guided by the predictions of existing (somewhat uncertain)stellar evolution calculations, we suggest that this results fromprimordial enrichment from stars having mass as low as 1-3M_{sun}. (6) If the preceding suggestion is correct, chemicalenrichment in {ω} Cen occurred over an extended period, perhaps{\ge}1 Gyr.