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

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

2007-03-09

arxiv.org/abs/astro-ph/0703208v2

Astrophys.J.663:296-314,2007

Astronomy and Astrophysics

Astrophysics

31 pages, 19 figures, in press in ApJ. Replacement of astro-ph/0703208 in order to show figs. 1,2,3,4,and 6 in higher resoluti

Scientific paper

10.1086/517905

We present metallicity measurements based on GIRAFFE@VLT spectra of 80 subgiant-branch stars of the Galactic globular cluster Omega Centauri. The VLT spectroscopic data are complemented by ACS/HST and WFI@ESO2.2m high-accuracy color-magnitude diagrams. We have obtained the [Fe/H] abundance for each of the 80 target stars, and the abundances of C, N, Ca, Ti, and Ba for a subset of them, by comparison with synthetic spectra. We show that 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. We have identified four distinct stellar groups within the SGB region: (i) an old, metal-poor group ([Fe/H]~-1.7); (ii) an old, metal-rich group ([Fe/H]~-1.1); (iii) a young (up to 4--5 Gyr younger than the old component) metal-poor group ([Fe/H]~-1.7); (iv) 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, a group of SGB stars are spread between the intermediate-metallicity and metal-rich branches of the SGB. The spread in age within each population establishes that the progenitor of Omega Cen system must have had a composite nature.

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