A LARGE SAMPLE STUDY OF RED GIANTS IN THE GLOBULAR CLUSTER OMEGA CENTAURI (NGC 5139)

We present abundances of several light, α, Fe-peak, and neutron-capture elements for 66 red giant branch (RGB) stars in the Galactic globular cluster Omega Centauri (ω Cen). Our observations lie in the range 12.0 < V < 13.5 and focus on the intermediate and metal-rich RGBs. Abundances were determined using equivalent width measurements and spectrum synthesis analyses of moderate resolution (R ∼ 18,000) spectra obtained with the Blanco 4 m telescope and Hydra multifiber spectrograph. Combining these data with previous work, we find that there are at least four peaks in the metallicity distribution function at [Fe/H] = -1.75, -1.45, -1.05, and -0.75, which correspond to about 55%, 30%, 10%, and 5% of our sample, respectively. Additionally, the most metal-rich stars are the most centrally located. Na and Al are correlated despite exhibiting star-to-star dispersions of more than a factor of 10, but the distribution of those elements appears to be metallicity dependent and are divided at [Fe/H] ∼-1.2. About 40%-50% of stars with [Fe/H] <-1.2 have Na and Al abundances consistent with production solely in Type II supernovae and match observations of disk and halo stars at comparable metallicity. The remaining metal-poor stars are enhanced in Na and Al compared to their disk and halo counterparts and are mostly consistent with predicted yields from >5 M sun asymptotic giant branch (AGB) stars. At [Fe/H] > - 1.2, more than 75% of the stars are Na/Al enhanced and may have formed almost exclusively from AGB ejecta. Most of these stars are enhanced in Na by at least 0.2 dex for a given Al abundance than would be expected based on 'normal' globular cluster values. All stars in our sample are α-rich with ([Ca/Fe]) = +0.36 (σ = 0.09) and ([Ti/Fe]) = +0.23 (σ = 0.14). The Fe-peak elements give solar-scaled abundances and similarly small dispersions with ([Sc/Fe]) = +0.09 (σ = 0.15) and ([Ni/Fe]) = -0.04 (σ = 0.09). Europium does not vary extensively as a function of metallicity and has ([Eu/Fe]) = +0.19 (σ = 0.23). However, [La/Fe] varies from about -0.4 to +2 and stars with [Fe/H] ∼>-1.5 have [La/Eu] values indicating domination by the s-process. A quarter of our sample have [La/Eu] ≥ +1 and may be the result of mass transfer in a binary system. We conclude that the metal-rich population must be at least 1-2 Gyr younger than the metal-poor stars, owing to the long timescales needed for strong s-process enrichment and the development of a large contingent of mass transfer binaries.