The formation of barium and CH stars and related objects

Abstract

We investigate the formation of barium (Ba) and CH stars via binary interactions. We consider four evolutionary channels for their formation: wind accretion, wind exposure, stable Roche lobe overflow (RLOF), and common-envelope (CE) ejection, and carry out Monte Carlo simulations. We explore the effects of model parameters, such as a tidal-enhancement parameter for stellar wind, a maximum stellar mass for s-processing, and a minimum core mass for thermal pulsation, on our results. We also explore the effects of various assumptions about age, mass-ratio distribution and wind velocity. Our results show that binary interaction is successful in explaining the formation of Ba and CH stars. We successfully explain the distribution of orbital periods, the distribution of mass functions, and the number of observed Ba stars. We also support the views (a) that a tidally enhanced stellar wind exists, (b) that thermal pulsation may begin at an earher phase during the asymptotic giant branch (AGB) than is usually assumed, and (c) that the maximum stellar mass for s-processing is about 2M 0. We find that the degree of Ba pollution is strongly correlated with orbital period. The average mass of strong Ba stars is estimated to be 1.8 M 0 , and of all Ba stars 1.7 M 0 ; the average mass for the white dwarf (WD) companions in the Ba-star binaries is estimated to be 0.60 M 0. The average mass of strong CH stars is estimated to be 1.2 M 0 , and the average mass for their WD companions is estimated to be 0.62 M 0. The total number of Ba stars in the Galaxy brighter than 10th apparent magnitude is estimated to be 6000, while that of CH stars is much less. We also investigate the formation of pre-Ba/CH stars, cataclysmic variables, Algols, double degenerates and symbiotics. Some encouraging results are found, although much work needs to be done in order to understand fully their formation, especially of pre-Ba/CH stars and of symbiotics.