A SPECTROSCOPIC ANALYSIS OF BARIUM STARS

Detailed abundance analyses have been carried out for 31 barium and no rmal G-K giant stars using high-dispersion spectra and model atmospher es. A significant enhancement of s-process elements is found for seven teen stars. The abundances of light and iron-peak elements are in gene ral equal to those in the standard star epsilon Vir. However, Na, Mg, Mn, and Co are systematically slightly deficient (about 0.2 dex). The elements heavier than Ni are enhanced by up to about 1.5 dex compared with the standards, while the r-process element Eu abundance is roughl y normal. We cannot find substantial differences in abundances, atmosp heric parameters, and luminosities of radial- velocity variable and no nvariable barium stars. Therefore it seems that both groups of stars b elong to a single family of peculiar giants. Comparison between the me an observed s-process abundances for our uniform barium star sample, a nd theoretical predictions from various neutron exposure models, show that C-13 neutron source AGB star (with the mean neutron exposure tau0 > 0.4 mb-1) can best reproduce the abundance data of these stars. Low neutron density single neutron exposures of approx. 1.1 mb-1 also are shown to result in good agreement with the barium star observations. Mass transfer scenarios are tested using the chemical composition and orbital parameter data of Ba II stars. Since a correlation exists betw een s-process abundance anomalies and orbital periods (projections of the semi-major axis, mass functions) for barium star binaries, we conc lude that a wind accretion scenario is more promising than Roche-lobe overflow ones. Abundance patterns for barium and carbon stars have bee n compared. We find good agreement for the iron group metals, but carb on stars show higher s-process element overabundances (0.9 dex in the mean). Therefore, the companions to the Ba II stars were perhaps once carbon stars, who overflowed mass onto the presently visible star with a second dilution (the ratio of the transferred mass to the mass of t he receiving envelope) of roughly 0.9.