The Carina Project VII: Towards the breaking of the age-metallicity degeneracy of red giant branch stars using the c_UBI index

TL;DR

This study introduces a new photometric index, c_UBI, which effectively separates and analyzes the age and metallicity of red giant branch stars in the Carina dwarf galaxy, providing insights into its chemical evolution.

Contribution

The paper presents a novel pseudo-colour index, c_UBI, that disentangles age and metallicity in RGB stars, improving understanding of galaxy evolution and breaking the age-metallicity degeneracy.

Findings

c_UBI effectively separates old and intermediate-age populations.

Old stars are more metal-poor ([Fe/H]=-2.32) than intermediate-age stars ([Fe/H]=-1.82).

Theoretical models do not fully match observed distributions in the c_UBI plane.

Abstract

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.