Red clump stars of the Milky Way - laboratories of extra-mixing

In this work we present the main atmospheric parameters, carbon, nitrogen and oxygen abundances, and C-12/(13)Cratios determined in a sample of 28 Galactic clump stars. Abundances of carbon were studied using the C-2 band at 5086.2 angstrom. The wavelength interval 7980-8130 angstrom with strong CN features was analysed in order to determine nitrogen abundances and C-12/C-13 isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 angstrom. The mean abundances of C, N and O abundances in the investigated clump stars support our previous estimations that, compared to the Sun and dwarf stars of the Galactic disc, carbon is depleted by about 0.2 dex, nitrogen is enhanced by 0.2 dex and oxygen is close to abundances in dwarfs. The C-12/C-13 and C/N ratios for galactic red clump stars analysed were compared to the evolutionary models of extra-mixing. The steeper drop of C-12/C-13 ratio in the model of thermohaline mixing by Charbonnel & Lagarde better reflects the observational data at low stellar masses than the more shallow model of cool bottom processing by Boothroyd & Sackmann. For stars of about 2M(circle dot) masses a modelling of rotationally induced mixing should be considered with rotation of about 250 km s(-1) at the time when a star was at the hydrogen-core-burning stage.