CNONa and C/C in giant stars of 10 open clusters ^*,**,***

¹ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, 05508-090, São Paulo, SP, Brazil e-mail: [rodolfo;barbuy]@astro.iag.usp.br
² Laboratoire d'astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland e-mail: Pierre.North@epfl.ch
³ Geneva Observatory, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland e-mail: Corinne.Charbonnel@unige.ch
⁴ LATT, CNRS UMR 5572, Université de Toulouse, 14 avenue Edouard Belin, 31400 Toulouse Cedex 04, France
⁵ Observatoire de Genève, Integral Science Data Center, Chemin d'Ecogia 16, 1290 Versoix, Switzerland e-mail: Nami.Mowlavi@unige.ch

Received: 9 October 2008
Accepted: 10 April 2009

Abstract

Context. Evolved low-mass stars (0.8 ≤ ≤ 2.5) of a wide range of metallicity bear signatures of a non-standard mixing event in their surface abundances of Li, C, and N, and in their ¹²C/¹³C ratio. A Na overabundance has also been reported in some giants of open clusters but remains controversial. The cause of the extra-mixing has been attributed to thermohaline convection that should take place after the RGB bump for low-mass stars and on the early-AGB for more massive objects.

Aims. To track the occurrence of this process over a wide mass range, we derive in a homogeneous way the abundances of C, N, O, and Na, as well as the ¹²C/¹³C ratio in a sample of 31 giants of 10 open clusters with turn-off masses from 1.7 to 3.1 . The sample includes red giants, clump giants, and early-AGB stars. We study the observational behavior of the abundances as well as the possible correlations between different elements and between the chemical abundances and stellar mass.

Methods. A model atmosphere analysis is conducted using high signal-to-noise ratio, high-resolution FEROS and EMMI spectra. We derive atmospheric parameters using Fe i and Fe ii lines. We calculate abundances for Na, C, N, and O, as well as the ¹²C/¹³C ratio using spectral synthesis. For the elements Mg, Ca, Si, Sc, Ti, V, Cr, Co, and Ni, abundances are derived using equivalent widths.

Results. A group of first ascent red giants with ≤ 2.5 exhibits lower [N/C] ratios than those measured in clump giants of the same mass range, suggesting an additional increase in the [N/C] ratio after the first dredge-up. The sodium abundances corrected from NLTE are found to be about solar. [Na/Fe] shows a slight increase of 0.10 dex as a function of stellar mass in the 1.8 to 3.2 range covered by our sample, in agreement with standard first dredge-up predictions. Our results do not support previous claims of sodium overabundances as high as +0.60 dex. An anti-correlation between ¹²C/¹³C and turn-off mass is identified and interpreted as being caused by a post-bump thermohaline mixing. Moreover, we find low ¹²C/¹³C ratios in a few intermediate-mass early-AGB stars, confirming that an extra-mixing process also operates in stars that do not experienced the RGB bump. In this case, the extra-mixing possibly acts on the early-AGB, in agreement with theoretical expectations for thermohaline mixing.