The Abundance Spread Among Giants and Subgiants in the Globular Cluster Omega Centauri
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We present spectroscopic abundances and radial velocities for giant stars in the Galactic globular cluster Centauri based on the Ca II infrared triplet. Two samples of stars were observed: 234 stars at MV1.25 on the lower giant branch at radial distances between 8 and 23, and 145 stars at MV - 1.3 at radial distances between 3 and 22 . We found 199 and 144 radial velocity members, respectively, in the two samples. These samples were corrected for evolutionary effects to provide an unbiased distribution of the underlying stellar metallicity. We find r = 234.7 1.3, obs= 11.3 km s-1 (bright sample), and r = 232.91.2, obs = 10.6 km s-1 (faint sample). The statistical errors of the dispersions are less than 1 km s-1. Previous metallicity studies found a non-Gaussian metallicity distribution containing a tail of metal-rich stars. We confirm these results except our unbiased cluster metallicity distributions are narrower. They contain the following key features: (1) No very metal-poor stars, (2) a sudden rise in the metal-poor distribution to a modal [Fe/H] value of - 1.70 consistent with an homogeneous metallicity unresolved at the 0.07 dex level, (3) a tail to higher metallicities with more stars than predicted by simple chemical evolution models, and (4) a weak correlation between metallicity and radius such that the most metal-rich stars are concentrated to the cluster core. The unresolved metal-weak tail implies that the gas out of which Cen formed was well-mixed up to the modal metallicity of the cluster. Therefore, Cen like other Galactic globular clusters, seems to have formed in a pre-enriched and homogenized (up to the modal metallicity) environment. The existence of a weak metallicity gradient supports the idea that Cen self-enriched, with the enriched gas sinking to the cluster center due to gas dissipation processes. We also note, however, that the metal-rich stars are more massive than the bulk of the stars in the cluster, and could also have sunk to the center by dynamical mass segregation over the lifetime of the cluster. 1996 American Astronomical Society.