The Multiplicity of the Subgiant Branch of ω Centauri: Evidence for Prolonged Star Formation

We combine spectroscopic and photometric data for subgiant stars of ω Cen to extract results that neither data set could have provided on its own. GIRAFFE@VLT spectra of 80 stars at R = 6400 give metallicities for all of them and abundances of C, N, Ca, Ti, and Ba for a subset of them. The photometric data, which have unusually high accuracy, come from a ~10 × 10 arcmin2 mosaic of HST ACS images centered on the cluster center and on multicolor images of a ~34 × 33 arcmin2 field, taken with the WFI@ESO2.2m camera. Stars with [Fe/H] < -1.25 have a large magnitude spread on the flat part of the SGB. We interpret this as empirical evidence for an age spread, and from theoretical isochrones we derive a relative age for each star. Within the SGB region we identify four distinct stellar groups: (1) an old, metal-poor group ([Fe/H] ~ -1.7); (2) an old, metal-rich group ([Fe/H] ~ -1.1); (3) a young (up to 4-5 Gyr younger than the old component) metal-poor group ([Fe/H] ~ -1.7); (4) a young, intermediate-metallicity ([Fe/H] ~ -1.4) group, on average 1-2 Gyr younger than the old metal-poor population, and with an age spread that we cannot properly quantify with the present sample. In addition, many SGB stars are spread between the intermediate-metallicity and metal-rich branches. We tentatively propose connections between the SGB stars and both the multiple main-sequence and the red giant branches. Finally, we discuss the implications of the multiple stellar populations on the formation and evolution of ω Cen. The spread in age within each population establishes that the original system must have had a composite nature.