Chemical composition of clump stars in the open cluster NGC 6134★

Abstract

We present an analysis of high-resolution spectra of six core-helium-burning ‘clump’ stars in the open cluster NGC 6134. Atmospheric parameters (Teff, log g, vt and [Fe/H]) were determined in our previous study by Carretta et al. (2004). In this study we present abundances of C, N, O and up to 24 other chemical elements. 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 in order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen abundances were determined from the [O i] line at 6300 Å. Compared to the Sun and other dwarf stars of the Galactic disc, mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.2 dex, nitrogen is overabundant by about 0.3 dex and oxygen is underabundant by about 0.1 dex. This has the effect of lowering the mean C/N ratio to 1.2 ± 0.2. The mean 12C/13C ratios are lowered to 9 ± 2.5. Concerning other chemical elements, the analysis of sodium and magnesium lines, lines of other α-elements, iron-group and heavier chemical elements gave abundance ratios close to the solar ones.