Volume 541, May 2012
|Number of page(s)||7|
|Published online||16 May 2012|
C, N, O abundances and carbon isotope ratios in evolved stars of the open clusters Collinder 261 and NGC 6253 ⋆
1 Institute of Theoretical Physics and Astronomy, Vilnius University, A. Gostauto 12, 01108 Vilnius, Lithuania
e-mail: firstname.lastname@example.org; email@example.com
2 INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
3 INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: firstname.lastname@example.org; email@example.com
Received: 17 January 2012
Accepted: 19 March 2012
Context. Investigations of abundances of carbon and nitrogen in the atmospheres of evolved stars of open clusters may provide comprehensive information on chemical composition changes caused by stellar evolution.
Aims. Our main aim is to increase the number of open clusters with determined carbon-to nitrogen and carbon isotope ratios.
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 (FEROS spectra) and the C2 Swan (1, 0) band head at 4737 Å (UVES spectra). The wavelength interval 7980–8130 Å, with strong CN features was analysed to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen abundances were determined from the [O i] line at 6300 Å.
Results. The average value of 12C/13C isotope ratios of Cr 261 is equal to 18 ± 2 in four giants and to 12 ± 1 in two clump stars; it is equal to 16 ± 1 in four clump stars of the open cluster NGC 6253. The mean C/N ratios in Cr 261 and NGC 6253 are equal to 1.67 ± 0.06 and 1.37 ± 0.09, respectively.
Conclusions. The 12C/13C and C/N values in Cr 261 and NGC 6253 within limits of uncertainties agree with the theoretical model of thermohaline-induced mixing as well as with the cool-bottom processing model.
Key words: stars: abundances / stars: horizontal-branch / open clusters and associations: individual: collinder 261 / open clusters and associations: individual: NGC 6253
© ESO, 2012
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