2016A&A...595A..16T

Aims. 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 2M_☉, and to compare them with predictions of theoretical models.
Methods. 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 5635.5Å. The wavelength interval 7940-8130Å with strong CN features was analysed to determine nitrogen abundances and carbon isotope ratios. The oxygen abundances were determined from the [OI] line at 6300Å.
Results. 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.
Conclusions. 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.