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We analysed high-resolution UVES spectra of six stars belonging to thesubgiant branch of omega Centauri, and derived abundance ratios of 19 chemicalelements (namely Al, Ba, C, Ca, Co, Cr, Cu, Fe, La, Mg, Mn, N, Na, Ni, Sc, Si,Sr, Ti, and Y). A comparison with previous abundance determinations for redgiants provided remarkable agreement and allowed us to identify thesub-populations to which our targets belong. We found that three targets belongto a low-metallicity population at [Fe/H]~-2.0 dex, [alpha/Fe]~+0.4 dex and[s/Fe]~0 dex. Stars with similar characteristics were found in small amounts bypast surveys of red giants. We discuss the possibility that they belong to aseparate sub-population that we name VMP (very metal-poor, at most 5% of thetotal cluster population), which - in the self-enrichment hypothesis - is thebest-candidate first stellar generation in omega Cen. Two of the remainingtargets belong to the dominant metal-poor population (MP) at [Fe/H]~-1.7 dex,and the last one to the metal-intermediate (MInt) one at [Fe/H]~-1.2 dex. Theexistence of the newly defined VMP population could help to understand somepuzzling results based on low-resolution spectroscopy (Sollima et al.,Villanova et al.) in their age differences determinations, because themetallicity resolution of these studies was probably not enough to detect theVMP population. The VMP could also correspond to some of the additionalsubstructures of the subgiant-branch region found in the latest HST photometry(Bellini et al.). After trying to correlate chemical abundances withsubstructures in the subgiant branch of omega Cen, we found that the agedifference between the VMP and MP populations should be small (0+/-2 Gyr),while the difference between the MP and MInt populations could be slightlylarger (2+/-2~Gyr).

The subgiant branch of omega Cen seen through high-resolution spectroscopy. I. The first stellar generation in omega Centauri?