Chemical analysis of carbon stars in the Local Group

I. The Small Magellanic Cloud and the Sagittarius dwarf spheroidal galaxy

¹ Observatoire de la Côte d'Azur, Dpt. Cassiopée UMR6 202, 06 304 Nice Cedex 4, France e-mail: laverny@obs-nice.fr
² Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18 071 Granada, Spain
³ GRAAL, UMR5024, Université de Montpellier II, 34 095 Montpellier Cedex 5, France
⁴ INAF - Osservatorio di Collurania, 64 100 Teramo, Italy
⁵ Department of Astronomy and Space Physics, Box 515, 75120 Uppsala, Sweden
⁶ Niels Bohr Institute, Astronomical Observatory, Juliane Maries vej 30, 2 100 Copenhagen, Denmark

Received: 12 May 2005
Accepted: 12 September 2005

Abstract

We present the first results of our ongoing chemical study of carbon stars in the Local Group of galaxies. We used spectra obtained with UVES at the 8.2 m Kueyen-VLT telescope and a new grid of spherical model atmospheres for cool carbon-rich stars which include polyatomic opacities, to perform a full chemical analysis of one carbon star, BMB-B 30, in the Small Magellanic Cloud (SMC) and two, IGI95-C1 and IGI95-C3, in the Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy. Our main goal is to test the dependence on the stellar metallicity of the s-process nucleosynthesis and mixing mechanism occurring in AGB stars. For these three stars, we find important s-element enhancements with respect to the mean metallicity ([M/H]), namely , similar to the figure found in galactic AGB stars of similar metallicity. The abundance ratios derived between elements belonging to the first and second s-process abundance peaks, corresponding to nuclei with a magic number of neutrons (88Sr, 89Y, 90Zr) and (138Ba, 139La, 140Ce, 141Pr), agree remarkably well with the theoretical predictions of low mass metal-poor AGB nucleosynthesis models where the main source of neutrons is the ¹³CO reaction activated during the long interpulse phase, in a small pocket located within the He-rich intershell. The derived C/O and ¹²C/¹³C ratios are, however, more difficult to reconcile with theoretical expectations. Possible explanations, like the extrinsic origin of the composition of these carbon stars or the operation of a non-standard mixing process during the AGB phase (such as the cool bottom process), are discussed on the basis of the collected observational constraints.