The Carina Project VII: Towards the breaking of the age-metallicity
  degeneracy of red giant branch stars using the c_UBI index
  We present an analysis of photometric and spectroscopic data of the Carina
dSph galaxy, testing a new approach similar to that used to disentangle
multiple populations in Galactic globular clusters (GCs). We show that a proper
colour combination is able to separate a significant fraction of the red giant
branch (RGB) of the two main Carina populations (the old one, \sim 12 Gyr, and
the intermediate-age one, 4-8 Gyr). In particular, the c_UBI=(U-B)-(B-I)
pseudo-colour allows us to follow the RGB of both populations along a relevant
portion of the RGB. We find that the oldest stars have more negative c_UBI
pseudo-colour than intermediate-age ones. We correlate the pseudo-colour of RGB
stars with their chemical properties, finding a significant trend between the
iron content and the c_UBI. Stars belonging to the old population are
systematically more metal poor ([Fe/H]=-2.32\pm0.08 dex) than the
intermediate-age ones ([Fe/H]=-1.82\pm0.03 dex). This gives solid evidence on
the chemical evolution history of this galaxy, and we have a new diagnostic
that can allow us to break the age-metallicity degeneracy of H-burning advanced
evolutionary phases. We compared the distribution of stars in the c_UBI plane
with theoretical isochrones, finding that no satisfactory agreement can be
reached with models developed in a theoretical framework based on standard
heavy element distributions. Finally, we discuss possible systematic
differences when compared with multiple populations in GCs.
