The binary nature of the barium and CH stars. III - Orbital parameters

Abstract

Ba ii and CH stars are too faint to be located on the asymptotic giant branch (AGB). It is therefore unlikely that carbon and s-process elements could have been mixed up to the atmospheres of these objects by the helium shell flash mechanism. A program to monitor velocity variations in these stars over the last decade has resulted in the conclusions that all Ba n and CH stars are binaries. Very few have periods much longer than 10 yr, indicating that there is an upper limit for separations of the components of the binary systems. Orbits have now been obtained for 16 Ba n star and 8 CH star systems. The eccentricities of the Ba n star orbits are significantly lower, on the average (although not zero) than eccentricities for a sample of normal G and K giants. Furthermore, the eccentricities of the CH star orbits are found to be significantly lower than for the Ba ii stars. These low eccentricities provide evidence for dissipation due to mass exchange, probably from a previous AGB star. The mass functions for Ba n and CH stars also show very different distributions than do those for a sample of spectroscopically normal giants. They indicate that both the Ba n and CH stars come from samples of binary systems with a small dispersion in mass ratios. If the Ba n and CH stars are assumed to have masses of 1.5 and 0.8 M 0 , respectively, then the mass functions are such that their companions would have masses near 0.6 M 0 similar to values expected for white dwarfs.