The extent of mixing in stellar interiors: the open clusters Collinder 261 and Melotte 66
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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- May 2016
- DOI:
- 10.1051/0004-6361/201628138
- arXiv:
- arXiv:1603.09529
- Bibcode:
- 2016A&A...589A..50D
- Keywords:
-
- stars: abundances;
- open clusters and associations: individual: Collinder 261;
- stars: horizontal-branch;
- stars: evolution;
- open clusters and associations: individual: Melotte 66;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 6 pages, 8 figures