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 12C/13C ratios determined in a sample of 28 Galactic
clump stars. Abundances of carbon were studied using the C2 band at 5086.2
{\AA}. The wavelength interval 7980-8130 {\AA} 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 {\AA}. 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 12C/13C and C/N ratios
for galactic red clump stars analysed were compared to the evolutionary models
of extra-mixing. The steeper drop of 12C/13C 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 2 Msun 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.
