The Discovery of an Anomalous Subgiant Branch in the Color-Magnitude Diagram of Centauri

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Abstract
Using deep high-resolution multiband images taken with the Very Large Telescope and the Hubble Space Telescope, we discovered a new anomalous sequence in the color-magnitude diagram of ω Cen. This feature appears as a narrow well-defined subgiant branch (SGB-a), which merges into the main sequence of the dominant cluster population at a magnitude significantly fainter than the cluster turnoff (TO). The simplest hypothesis assumes that the new feature is the extension of the anomalous red giant branch (RGB-a) metal-rich population discovered by Lee et al. and Pancino et al. However, under this assumption the interpretation of the SGB-a does not easily fit into the context of a self-enrichment scenario within ω Cen. In fact, its TO magnitude, shape, and extension are not compatible with a young metal-rich population, as required by the self-enrichment process. The TO level of the SGB-a suggests indeed an age as old as the main cluster population, further supporting the extracluster origin of the most metal-rich stars, as suggested by Ferraro, Bellazzini, & Pancino. Only accurate measurements of radial velocities and metal abundances for a representative sample of stars will firmly establish whether or not the SGB-a is actually related to the RGB-a and will finally shed light on the origin of the metal-rich population of ω Cen.