The Chemical Evolution of the Globular Cluster ω Centauri (NGC 5139)

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Abstract
We present abundances for 22 chemical elements in 10 red giant members of the massive Galactic globular cluster ω 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 ω Cen have [Fe/H] ~ -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] ~ -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 AGB stars, the dominant neutron source is 13C(α, n)16O. A comparison of the Rb/Zr abundance ratios in ω Cen finds that these ratios are consistent with the 13C source. The reason ω Cen displays such a large s-process component is possibly due to the fact that in such a relatively low-mass stellar system, AGB ejecta, because of their low velocity winds, are more efficiently retained in the cluster relative to the much faster moving Type II supernova ejecta. Significant s-process enrichment relative to Fe, from the lower mass AGB 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 AGB ejecta were mixed with the retained fraction of Type II supernova ejecta and with the residual gas of initial composition. The analysis of α-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] ~ -0.6 (derived here for the first time in this cluster) finds a plausible interpretation.