  High-resolution spectra of 13 core helium-burning stars in the thick disk of
the Galaxy have been obtained with the SOFIN spectrograph on the Nordic Optical
Telescope to investigate abundances of up to 22 chemical elements. Abundances
of carbon were studied using the C_2 Swan (0,1) band head at 5635.5 A. The
wavelength interval 7980-8130 A 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 A. Abundances in the
investigated stars suggest that carbon is depleted by about 0.3 dex, nitrogen
is enhanced by more than 0.4 dex and oxygen is unaltered. The 12C/13C ratios
are lowered and lie between values 3 and 7 which is in agreement with "cool
bottom processing" predictions (Boothroyd & Sackmann 1999). The C/N ratios in
the investigated stars are lowered to values between 0.7 and 1.2 which is less
than present day theoretical predictions and call for further studies of
stellar mixing processes. Abundance ratios of O, Mg, Eu and other heavy
chemical elements to iron in the investigated stars show a pattern
characteristic of thick disk stars. The results provide evidence that the thick
disk population has a distinct chemical history from the thin disk. The onset
of the bulk of SN Ia is suggested to appear around [Fe/H] = -0.6 dex.
