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http://hdl.handle.net/20.500.12386/24877
Titolo: | The extent of mixing in stellar interiors: the open clusters Collinder 261 and Melotte 66 | Autori: | Drazdauskas, Arnas Tautvaišienė, Gražina RANDICH, Maria Sofia BRAGAGLIA, Angela Mikolaitis, Šarūnas Janulis, Rimvydas |
Data pubblicazione: | 2016 | Rivista: | ASTRONOMY & ASTROPHYSICS | Numero: | 589 | Da pagina:: | A50 | 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 C2Swan (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. <BR /> 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 <SUP>12</SUP>C /<SUP>13</SUP>C 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 <SUP>12</SUP>C /<SUP>13</SUP>C and C/N ratios are 8 ± 2 and 1.67 ± 0.21, respectively. Conclusions: The <SUP>12</SUP>C /<SUP>13</SUP>C 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 <SUP>12</SUP>C /<SUP>13</SUP>C 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<SUB>☉</SUB>. 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. Based on observations collected at ESO telescopes under Guaranteed Time Observation programmes 071.D-0065, 072.D-0019, and 076.D-0220. | URI: | http://hdl.handle.net/20.500.12386/24877 | URL: | https://www.aanda.org/articles/aa/abs/2016/05/aa28138-16/aa28138-16.html | ISSN: | 0004-6361 | DOI: | 10.1051/0004-6361/201628138 | Bibcode ADS: | 2016A&A...589A..50D | Fulltext: | open |
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