CNO abundances and carbon isotope ratios in evolved stars of the open clusters NGC 2324, NGC 2477, and NGC 3960

TL;DR

This study measures CNO abundances and carbon isotope ratios in evolved stars of three open clusters to test stellar evolution models, finding good agreement with models involving mixing processes like thermohaline and rotation-induced mixing.

Contribution

It provides new observational data on CNO and carbon isotope ratios in open cluster giants, comparing them with theoretical models of stellar mixing processes.

Findings

Ratios agree with first dredge-up and thermohaline models for NGC 2324 and NGC 2477.

Carbon isotope ratios in NGC 3960 suggest combined thermohaline and rotation-induced mixing.

Observed ratios support current stellar evolution theories involving mixing processes.

Abstract

Our main aim is to determine carbon-to-nitrogen and carbon isotope ratios for evolved giants in the open clusters NGC 2324, NGC 2477, and NGC 3960, which have turn-off masses of about 2 Msun, and to compare them with predictions of theoretical models. High-resolution spectra were analysed using a differential synthetic spectrum method. Abundances of carbon were derived using the C2 Swan (0,1) band heads at 5135 and 5635.5 A. The wavelength interval 7940-8130 A with strong CN features was analysed to determine nitrogen abundances and carbon isotope ratios. The oxygen abundances were determined from the [Oi] line at 6300 A. The mean values of the CNO abundances are [C/Fe]=-0.35+-0.06 (s.d.), [N/Fe]=0.28+-0.05, and [O/Fe]=-0.02+-0.10 in seven stars of NGC 2324; [C/Fe]=-0.26+-0.02, [N/Fe]=0.39+-0.04, and [O/Fe]=-0.11+-0.06 in six stars of NGC 2477; and [C/Fe]=-0.39+-0.04, [N/Fe]=0.32+-0.05, and [O/Fe]=-0.19+-0.06 in six stars of NGC 3960. The mean C/N ratio is equal to 0.92+-0.12, 0.91+-0.09, and 0.80+-0.13, respectively. The mean 12C/13C ratio is equal to 21+-1, 20+-1, and 16+-4, respectively. The 12C/13C and C/N ratios of stars in the investigated open clusters were compared with the ratios predicted by stellar evolution models. The mean values of the 12C/13C and C/N ratios in NGC 2324 and NGC 2477 agree well with the first dredge-up and thermohaline-induced extra-mixing models, which are similar for intermediate turn-off mass stars. The 12C/13C ratios in the investigated clump stars of NGC 3960 span from 10 to 20. The mean carbon isotope and C/N ratios in NGC 3960 are close to predictions of the model in which the thermohaline- and rotation-induced (if rotation velocity at the zero-age main sequence was 30% of the critical velocity) extra-mixing act together.