Carbon and nitrogen as indicators of stellar evolution and age

A homogeneous sample of 44 open clusters from the Gaia-ESO Survey^⋆

¹ Vilnius University, Faculty of Physics, Institute of Theoretical Physics and Astronomy, Saulėtekio av. 3, 10257 Vilnius, Lithuania
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 3, 50125 Firenze, Italy
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, via P. Gobetti 93/3, 40129 Bologna, Italy
⁴ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
⁵ Research School of Astronomy & Astrophysics, Australian National University, Cotter Rd., Weston, ACT 2611, Australia
⁶ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO3D), Stromlo, Australia

^⋆⋆ Corresponding author: grazina.tautvaisiene@tfai.vu.lt

Received: 27 May 2025
Accepted: 28 August 2025

Abstract

Context. Low- and intermediate-mass giants undergo a complex chemical evolution that has yet to be observationally probed. The influence of core helium flash on the chemical composition of stellar atmospheres has been an open question since its theoretical prediction 60 years ago.

Aims. Based on high-resolution spectral observations of 44 open star clusters in the Gaia-ESO survey, our aim is to perform the first large-scale homogeneous investigation into the carbon and nitrogen photospheric content of low- and intermediate-mass giant stars in different phases of evolution.

Methods. We determined carbon and nitrogen abundances using spectral synthesis of the C₂ Swan (1,0) band head at 5135 Å and C₂ Swan (0,1) band head at 5635.5 Å, ¹²C¹⁴N bands in the interval 6470–6490 Å, and the forbidden [O I] line at 6300.31 Å.

Results. We revealed differences in C/N abundance ratios between pre- and post-core-He-flash stars. The lower C/N ratios in core He-burning red clump stars are mainly due to the enhancement of nitrogen abundances. We presented calibrations of the relationship between [C/N] and stellar age for solar metallicity low- and intermediate-mass giants taking into account different evolutionary stages.

Conclusions. The C/N abundance ratios in the investigated first-ascent giant stars are slightly less affected by the first dredge-up than predicted by the theoretical models. The rotation-induced extra mixing is not as efficient as theoretically predicted. The core helium flash may trigger additional alterations in carbon and nitrogen abundances that are not yet theoretically modelled. We found that the evolutionary stage of stars must be taken into account when using [C/N] as an age indicator.