Carbon and nitrogen as indicators of stellar evolution and age. A homogeneous sample of 44 open clusters from the Gaia-ESO Survey

TL;DR

This study provides the first large-scale, homogeneous assessment of carbon and nitrogen surface abundances in low- and intermediate-mass giants across 44 open clusters from the Gaia-ESO Survey. By performing spectral synthesis of C2 and CN features and the [O I] line on a common atmospheric-parameter framework, the authors derive $[\mathrm{C/H}]$, $[\mathrm{N/H}]$, and $[\mathrm{C/N}]$ for 327 evolved stars and classify stars into BB, AB, and RC phases to trace abundance changes through RGB evolution and core He ignition. The results show that $[\mathrm{C/N}]$ evolves with evolutionary stage, with RC stars exhibiting the strongest depletion largely due to nitrogen enhancement; rotation-induced mixing is less efficient than some models predict, and the He-flash appears to induce additional, currently not fully modeled abundance alterations. The authors derive stage-specific $[\mathrm{C/N}]$–age relations applicable to solar-metallicity giants in roughly $1$–$4\,M_\odot$ and ages $0.1$–$7$ Gyr, emphasizing that using $[\mathrm{C/N}]$ as an age indicator requires accounting for evolutionary stage. Overall, the work provides robust empirical calibrations and underscores gaps in mixing physics, with data publicly available through CDS.

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.

Figures (7)

• Figure 1: Positions of stars of several OCs in the log $g$ versus $T_{\rm eff}$ diagram together with PARSEC evolutionary sequences by bib35 corresponding to the OC age and metallicity. The RGB luminosity bump is highlighted by the grey thick stick. The red symbols mark the first ascent giants located below the red giant branch luminosity bump (BB), the blue symbols mark stars above the red giant branch luminosity bump (AB) where extra-mixing processes are acting, and the yellow symbols mark helium core burning stars (RC).
• Figure 2: Histograms of metallicity, age, and Galactocentric distances of the investigated open clusters with CNO abundances determined.
• Figure 3: Comparison of the averaged C/N ratios in OC stars at different evolutionary stages with theoretical models. The solid line represents the C/N ratios predicted by the model for stars at the 1DUP and the dashed line represents the minimal values in the model with TH, both taken from bib14. The dotted line represents the model including the thermohaline- and rotation-induced mixing acting together from Lagarde2012. In the upper panel, the red symbols mark the first ascent giants located below the red giant branch luminosity bump (BB). In the middle panel, the blue symbols mark stars above the red giant branch luminosity bump (AB) where extra-mixing processes are acting, and in the bottom panel, the yellow symbols mark core helium-burning stars (Clump).
• Figure 4: Differences in [C/H], [N/H], and C/N ratios for post- and pre-core-He-burning stars versus turn-off mass. From left to right the data for Berkeley 32, NGC 2141, NGC 6705, Berkeley 30, and NGC 6067 are plotted. See text for further explanations.
• Figure 5: [N/Fe] abundance ratios in 1DUP RGB stars below the luminosity bump (the red symbols) and in the red clump stars (the yellow symbols) as a function of turn-off mass in the same cluster, showing larger nitrogen abundances in the red clump stars.