The extent of mixing in stellar interiors: the open clusters Collinder 261 and Melotte 66^⋆

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

Received: 15 January 2016
Accepted: 2 March 2016

Abstract

Context. Determining carbon and nitrogen abundances in red giants provides useful diagnostics to test mixing processes in stellar atmospheres.

Aims. Our main aim is to determine carbon-to-nitrogen and carbon isotope ratios for evolved giants in the open clusters Collinder 261 and Melotte 66 and to compare the results with predictions of theoretical models.

Methods. High-resolution spectra were analysed using a differential model atmosphere method. Abundances of carbon were derived using the C₂ Swan (0, 1) band head at 5635.5 Å. The wavelength interval 7940–8130 Å, which contains 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 elemental abundances in Collinder 261, as determined from seven stars, are: [ C/Fe ] = −0.23 ± 0.02 (s.d.), [ N/Fe ] = 0.18 ± 0.09, [ O/Fe ] = −0.03 ± 0.07. The mean ¹²C /¹³C ratio is 11 ± 2, considering four red clump stars and 18 for one star above the clump. The mean C/N ratios are 1.60 ± 0.30 and 1.74, respectively. For the five stars in Melotte 66 we obtained: [ C/Fe ] = −0.21 ± 0.07 (s.d.), [ N/Fe ] = 0.17 ± 0.07, [ O/Fe ] = 0.16 ± 0.04. The ¹²C /¹³C and C/N ratios are 8 ± 2 and 1.67 ± 0.21, respectively.

Conclusions. 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 ¹²C /¹³C ratios in Collinder 261 and Melotte 66 agree well with models of thermohaline-induced extra-mixing for the corresponding stellar turn-off masses of about 1.1–1.2 M_⊙. The mean C/N ratios are not decreased as much as predicted by the model in which the thermohaline- and rotation-induced extra-mixing act together.