Abundances in globular cluster red giants. IV - M22 and Omega Centauri

Abstract

A detailed abundance analysis has been performed for three members of M22 and five of ω Cen. The three giants in M22 are chemically identical with [Fe/H] = —1.78 dex except for a range of Na and, less certainly, Ba larger than the observational and modeling errors. The ω Cen giants show a range of abundance for all elements, with the light elements and the rare earths (plus Ba) enhanced by a larger factor than the Fe peak nuclei in the more metal-rich ω Cen stars. The two most metal-rich ω Cen stars have vastly different Na/Mg and Al/Mg ratios and Sc abundances. Explosive nucleosynthesis fails to predict the Na/Mg observed ratios at large Al/Mg values. The deduced neutron excesses are η ≈ 10^(-2) and 5 x 10^(-4); the former is the largest known for any star. These high values of η require massive (M ≥ 12 M_⊙) supernova progenitors and may also produce via the s-process the strong enhancement of Ba and the rare earths seen in the metal-rich ω Cen giants. It is suggested that such massive supernovae were much rarer in the less dense halo field, leading to the observed differences in abundance patterns of globular cluster giants versus halo field dwarfs.

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