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

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 C-2 Swan (0, 1) band head at 5635.5 angstrom. The wavelength interval 7940-8130 angstrom, 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 angstrom. 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 C-12/C-13 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 C-12/C-13 and C/N ratios are 8 +/- 2 and 1.67 +/- 0.21, respectively. Conclusions. The C-12/C-13 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 C-12/C-13 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-circle dot. 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.