Chemical composition of red horizontal branch stars in the thick disk of the Galaxy

High-resolution spectra of 13 core helium-burning stars in the thick disk o f the Galaxy have been obtained with the SOFIN spectrograph on the Nordic O ptical Telescope to investigate abundances of up to 22 chemical elements. A bundances of carbon were studied using the C-2 Swan (0, 1) band head at 563 5.5 Angstrom. The wavelength interval 7980-8130 Angstrom with strong CN fea tures 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 a t 6300 Angstrom. Abundances in the investigated stars suggest that carbon i s depleted by about 0.3 dex, nitrogen is enhanced by more than 0.4 dex and oxygen is unaltered. The C-12/C-13 ratios are lowered and lie between value s 3 and 7 which is in agreement with "cool bottom processing" predictions ( Boothroyd & Sackmann 1999). The C/N ratios in the investigated stars are lo wered to values between 0.7 and 1.2 which is less than present day theoreti cal predictions and call for further studies of stellar mixing processes. A bundance ratios of O, Mg, Eu and other heavy chemical elements to iron in t he investigated stars show a pattern characteristic of thick disk stars. Th e results provide evidence that the thick disk population has a distinct ch emical history from the thin disk. The onset of the bulk of SN Ia is sugges ted to appear at [Fe/H] approximate to -0.6 dex.