  We analysed high-resolution UVES spectra of six stars belonging to the
subgiant branch of omega Centauri, and derived abundance ratios of 19 chemical
elements (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 red
giants provided remarkable agreement and allowed us to identify the
sub-populations to which our targets belong. We found that three targets belong
to 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 by
past surveys of red giants. We discuss the possibility that they belong to a
separate sub-population that we name VMP (very metal-poor, at most 5% of the
total cluster population), which - in the self-enrichment hypothesis - is the
best-candidate first stellar generation in omega Cen. Two of the remaining
targets 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. The
existence of the newly defined VMP population could help to understand some
puzzling results based on low-resolution spectroscopy (Sollima et al.,
Villanova et al.) in their age differences determinations, because the
metallicity resolution of these studies was probably not enough to detect the
VMP population. The VMP could also correspond to some of the additional
substructures of the subgiant-branch region found in the latest HST photometry
(Bellini et al.). After trying to correlate chemical abundances with
substructures in the subgiant branch of omega Cen, we found that the age
difference between the VMP and MP populations should be small (0+/-2 Gyr),
while the difference between the MP and MInt populations could be slightly
larger (2+/-2~Gyr).
