New Evidence for the Complex Structure of the Red Giant Branch in ω Centauri

Abstract

We report on the complex structure of the red giant branch (RGB) of ω Centauri, based on a new wide-field and wide-color baseline B and I photometry. Our color-magnitude diagram (CMD) shows the presence of multiple populations along this branch; in particular, we discovered an anomalous branch (RGB-a), which appears to be well separated from the bulk of the RGB stars. On the basis of our CMD and from the previous literature, we conclude that (1) these stars, clearly identified as a separate population in our CMD, represent the extreme metalrich extension ([Ca/H] > -0.3) of the stellar content of ω Cen and show anomalous abundances of s-process elements (as Ba and Zr) as well; (2) they are physical members of the ω Cen system; (3) they comprise ∼5% of the stars of the whole system; and (4) this component and the metal intermediate one (-0.4 > [Ca/H] > -1) have been found to share the same spatial distribution, both of them differing significantly from the most metalpoor one ([Ca/H] < -1 ). Th is last piece evidence supports the hypothesis that metal-rich components could belong to an independent (proto?) stellar system captured in the past by ω Cen.