The multiplicity of the subgiant branch of ω Centauri:: Evidence for prolonged star formation

We combine spectroscopic and photometric data for subgiant stars of omega Cen to extract results that neither data set could have provided on its own. GIRAFFE@ VLTspectra 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 similar to 10 x 10 arcmin(2) mosaic of HSTACS images centered on the cluster center and on multicolor images of a similar to 34 x 33 arcmin(2) 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] similar to -1.7); ( 2) an old, metal- rich group ([Fe/H] similar to -1.1); (3) a young ( up to 4-5 Gyr younger than the old component) metal-poor group ([Fe/H] similar to -1.7); (4) a young, intermediate-metallicity ([Fe/H] similar to -1.4) group, on average 1Y2 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 intermediatemetallicity 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 omega Cen. The spread in age within each population establishes that the original system must have had a composite nature.