THE FORMATION OF STARS WITH EXCESSES OF S-PROCESS ELEMENTS (BARIUM STARS) AS A RESULT OF THE EVOLUTION OF CLOSE BINARIES

The main properties (chemical composition, luminosity, frequency) of barium stars on the red giant branch and of the stars with barium star abundance peculiarities on the different parts of HR diagram are briefly reviewed. The calculations of simulated populations of barium stars are described. Mass transfer across a close binary system is the most plausible evolutionary scenario for stars with s-process element peculiarities. The main parameters of our calculations are the initial mass of the primary component, the initial mass ratio of the components, the semi-major axis of the orbit and the time of the binary star formation. The theoretical populations of s-process enriched stars (s-process enriched main-sequence stars, CH subgiants, barium stars) are generated using the <<population simulation>> technique. The described scenario is verv promising and quantitatively describes the barium star observational data. The frequency of s-process enriched stars as compared to the normal objects of similar luminosity and spectral type is about 5% for main-sequence stars and 5-6% for red giants. For about 1% of red giants an overabundance factor s/s0>3 (s/s0 almost-equal-to 3 the minimal level observable in red giants). There are very few stars on the main sequence with s/s0>10 (s/s0 almost-equal-to 10 is the minimal observable level for these stars). The most part of barium stars are formed according to the I case of the scenario (during mass exchange the secondary component is a main-sequence star). Several mass loss laws from the entire system were considered. at least 50% of the matter, that is lost from the primary component, has to be accreted by the secondary.