C and N abundances in globular clusters. I. The case of 47 Tuc and the effect of the first dredge-up

TL;DR

This study investigates chemical variations and multiple stellar populations in the globular cluster 47 Tuc, analyzing how C/N abundances evolve from the main sequence to the AGB and their impact on the color-magnitude diagram.

Contribution

It provides new insights into the evolution of C/N anti-correlation and the role of multiple populations in 47 Tuc, using homogeneous abundance measurements and CMD analysis.

Findings

MS stars are more C- and N-rich than evolved stars.

C/N anti-correlation shifts during the sub-giant branch phase.

Different populations occupy distinct CMD regions, requiring multiple isochrones.

Abstract

Globular clusters exhibit star-to-star chemical variations, traceable through both photometric and spectroscopic data. In this study, we investigate chemical variations in the globular cluster NGC104 (47Tucanae), aiming to trace multiple stellar populations across evolutionary phases and examine how the C/N anti-correlation evolves from the main sequence (MS) to the asymptotic giant branch (AGB). We also assess the impact of these populations on the interpretation of the V vs. V-I diagram. [C/Fe] and [N/Fe] abundances are obtained from CN and the CH features, while atmospheric parameters are homogeneously derived from photometry. The inferred abundances allow us to disentangle multiple populations along the CMD and refine cluster parameters. We find that MS stars are more C- and N-rich than their red giant branch, horizontal branch, and AGB counterparts. The C/N anti-correlation shifts during the sub-giant branch phase, coinciding with the first dredge-up. Interestingly, stars with different C and N abundances occupy distinct regions of the V vs V-I diagram, a pattern not attributable to differential reddening. Proper CMD fitting requires two isochrones with differing helium content, metallicity, and possibly age.