THE carina project. VII. Toward the breaking of the age-metallicity degeneracy of red giant branch stars using the C _U, B, i 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 color combination is able to separate a significant fraction of the red giant branch (RGB) of the two main Carina populations (the old one, 12 Gyr, and the intermediate-age one, 4-8 Gyr). In particular, the c _U, B, I = (U-B)-(B-I) pseudo-color allows us to follow the RGB of both populations along a relevant portion of the RGB. We find that the oldest stars have a more negative c _U, B, I pseudo-color than intermediate-age ones. We correlate the pseudo-color of RGB stars with their chemical properties, finding a significant trend between the iron content and the c _U, B, I. Stars belonging to the old population are systematically more metal-poor ([Fe/H] =-2.32 ± 0.08 dex) than the intermediate-age ones ([Fe/H] =-1.82 ± 0.03 dex). This gives solid evidence of 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 _U, B, I 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.