CHEMICAL ABUNDANCES FOR 855 GIANTS IN THE GLOBULAR CLUSTER OMEGA CENTAURI (NGC 5139)

Abstract

We present elemental abundances for 855 red giant branch (RGB) stars in the globular cluster Omega Centauri (ω Cen) from spectra obtained with the Blanco 4 m telescope and Hydra multifiber spectrograph. The sample includes nearly all RGB stars brighter than V = 13.5 and spans ω Cen's full metallicity range. The heavy α elements (Si, Ca, and Ti) are generally enhanced by ∼+0.3 dex and exhibit a metallicity-dependent morphology that may be attributed to mass and metallicity-dependent Type II supernova (SN) yields. The heavy α and Fe-peak abundances suggest minimal contributions from Type Ia SNe. The light elements (O, Na, and Al) exhibit >0.5 dex abundance dispersions at all metallicities, and a majority of stars with [Fe/H]> − 1.6 have [O/Fe], [Na/Fe], and [Al/Fe] abundances similar to those in monometallic globular clusters, as well as O–Na, O–Al anticorrelations and the Na–Al correlation in all but the most metal-rich stars. A combination of pollution from intermediate-mass asymptotic giant branch stars and in situ mixing may explain the light element abundance patterns. A large fraction (27%) of ω Cen stars are O-poor ([O/Fe] < 0) and are preferentially located within 5'–10' of the cluster center. The O-poor giants are spatially similar, located in the same metallicity range, and are present in nearly equal proportions to blue main-sequence stars. This suggests that the O-poor giants and blue main-sequence stars may share a common origin. [La/Fe] increases sharply at [Fe/H] ≳ −1.6, and the [La/Eu] ratios indicate that the increase is due to almost pure s-process production.

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