Elemental abundances in the atmosphere of clump giants

¹ Astronomical Observatory of Odessa National University and Isaac Newton Institute of Chile, Odessa Branch, Shevchenko Park, 65014 Odessa, Ukraine e-mail: tamar@deneb1.odessa.ua
² Observatoire Astronomique de l'Université Louis Pasteur, 11 rue de l'Université, 67000 Strasbourg, France
³ Geneva Observatory, 1290 Sauverny, Switzerland
⁴ LATT CNRS UMR 5572, 14 Av. E. Belin, 31400 Toulouse, France
⁵ Observatoire Aquitain des Sciences de l'Univers, CNRS UMR 5804, BP 89, 33270 Floirac, France

Received: 4 March 2006
Accepted: 24 May 2006

Abstract

Aims.The aim of this paper is to provide the fundamental parameters and abundances for a large sample of local clump giants with a high accuracy. This study is a part of a big project, in which the vertical distribution of the stars in the Galactic disc and the chemical and dynamical evolution of the Galaxy are being investigated.

Methods.The selection of clump stars for the sample group was made applying a colour-absolute magnitude window to nearby Hipparcos stars. The effective temperatures were estimated by the line depth ratio method. The surface gravities () were determined by two methods (the first one was the method based on the ionization balance of iron and the second one was the method based on fitting of the wings of the 6162.17 Å line). The abundances of carbon and nitrogen were obtained from the molecular synthetic spectrum, and the Mg and Na abundances were derived using the non-LTE approximation. The “classical" models of stellar evolution without atomic diffusion and rotation-induced mixing were employed.

Results.The atmospheric parameters (, , [Fe/H], ) and Li, C, N, O, Na, Mg, Si, Ca, and Ni abundances in 177 clump giants of the Galactic disc were determined. The underabundance of carbon, overabundance of nitrogen, and “normal” abundance of oxygen were detected. A small sodium overabundance was found. A possibility of a selection of the clump giants based on their chemical composition and the evolutionary tracks was explored.

Conclusions.The theoretical predictions based on the classical stellar evolution models are in good agreement with the observed surface variations of the carbon and nitrogen just after the first dredge-up episode. The giants show the same behaviour of the dependencies of O, Mg, Ca, and Si (α-elements) and Ni (iron-peak element) abundances vs. [Fe/H] as dwarfs do. This allows us to use such abundance ratios to study the chemical and dynamical evolution of the Galaxy.