The chemical evolution of the globular cluster ω Centauri (NGC 5139)

We present abundances for 22 chemical elements in 10 red giant members of the massive Galactic globular cluster omega Centauri. The spectra are of relatively high spectral resolution and signal-to-noise. Using these abundances plus published literature values, abundance trends are defined as a function of the standard metallicity indicator iron. The lowest metallicity stars in omega Cen have [Fe/H] similar to -1.8, and the initial abundance distribution in the cluster is established at this metallicity. The stars in the cluster span a range of [Fe/H] similar to -1.8 to -0.8. At the lowest metallicity, the heavy-element abundance is found to be well characterized by a scaled solar system r-process distribution, as found in other stellar populations at this metallicity. As iron increases, the s-process heavy-element abundances increase dramatically. Comparisons of the s-process increases with recent stellar models finds that s-process nucleosynthesis in 1.5-3 M-. asymptotic giant branch stars (AGB) fits well the heavy-element abundance distributions. In these low-mass AGE stars, the dominant neutron source is C-13(alpha,n)O-16. A comparison of the Rb/Zr abundance ratios in omega Cen finds that these ratios are consistent with the C-13 source. The reason omega Cen displays such a large s-process component is possibly due to the fact that in such a relatively low-mass stellar system, AGE ejects, because of their low velocity winds, are more efficiently retained in the duster relative to the much faster moving Type II supernova ejecta. Significant s-process enrichment relative to Fe, from the lower mass AGE stars, would require that the cluster was active in star formation for quite a long interval of time, of the order of 2-3 Gyr. The AGE ejecta were mixed with the retained fraction of Type II supernova ejecta and with the residual gas of initial composition. The analysis of alpha-rich elements shows that no significant amounts of Type Ia supernova debris were retained by the cluster. In this context, interpretation of the low and constant observed [Cu/Fe] similar to -0.6 (derived here for the first time in this cluster) finds a plausible interpretation.