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

¹ Institute of Theoretical Physics and Astronomy, Vilnius University, A. Gostauto 12, 01108 Vilnius, Lithuania
e-mail: grazina.tautvaisiene@tfai.vu.lt
² INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ INAF–Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

Received: 8 July 2016
Accepted: 25 August 2016

Abstract

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 2 M_⊙, 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 5635.5 Å. The wavelength interval 7940–8130 Å with strong CN features was analysed to determine nitrogen abundances and carbon isotope ratios. The oxygen abundances were determined from the [O i] line at 6300 Å.

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.