C and N abundances in globular clusters

I. The case of 47 Tuc (NGC 104) and the effect of the first dredge-up: Implications for the isochrone fitting

¹ Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Física y Astronomía – Instituto de Astrofísica, Autopista Concepción-Talcahuano 7100, Talcahuano, Chile
² INAF-Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
³ Osservatorio Astronomico di Padova-INAF, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Departamento de Astronomía, Casilla 160-C, Universidad de Concepción, Concepción 4030000, Chile
⁵ Las Cumbres Observatory 6740 Cortona Drive, Suite 102 Goleta, CA 93117, USA
⁶ Department of Geology, Sölvegatan 12, Lund, Sweden

^★ Corresponding author: sandro.villanova@unab.cl

Received: 14 March 2025
Accepted: 4 June 2025

Abstract

Context. Globular clusters exhibit star-to-star chemical variations that are traceable through both photometric and spectroscopic data. While UV photometry and light-elements such as Na and O are commonly used for this purpose, the optical V versus (V-I) color– magnitude diagram (CMD) is often assumed to be relatively unaffected by such inhomogeneities and is used to derive basic cluster parameters. On the other hand, C and N would be the best chemical tracers of these variations but are challenging to measure due to their spectral features lying in the blue/UV or IR regions.

Aims. In this study, we investigate chemical variations in the globular cluster NGC104 (47Tucanae) while aiming to trace multiple stellar populations across evolutionary phases and examining 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 versus V-I diagram.

Methods. Using spectra spanning all evolutionary stages, we derived [C/Fe] and [N/Fe] abundances for a large stellar sample. These abundance measurements were inferred from the CN and the CH features, while atmospheric parameters are homogeneously derived from photometry. The inferred abundances allowed us to disentangle multiple populations along the CMD and refine cluster parameters.

Results. We find that MS stars are more C and N-rich than their red giant branch, horizontal branch, and AGB counterparts. The C/N anticorrelation shifts during the sub-giant branch phase, coinciding with the first dredge-up, after which C decreases by 0.15–0.20 dex, N by ∼0.1 dex, while Fe remains unchanged. 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.