ON PREPLANETARY-NEBULA FORMATION

We propose a comprehensive model in which we examine the process of gas outflow by an evolved star (post-AGB stars) and the subsequent circumstellar shell formation. First, we investigate the buoyant transport of an isolated toroidlike tube through the red-giant convective envelope, originating from the innermost regions of the star and composed of strongly magnetized gas. When carrying up to a position immediately above the stellar surface, such a tube is no longer confined by the gas pressure existing throughout the red-giant envelope and consequently expands. It is hypothesized that the confluence of such magnetized tubes can produce a disklike structure of cool matter which surrounds the star. We have also analyzed the blowing process by a fast wind of the early ejected cool matter. The large- and small-scale structures of the preplanetary nebula shell such as the bipolarity, the filamentary pattern, and the multiple-shell appearance are then discussed. A critical comparison with other concurrent models (Balick et al. 1987; Soker and Livio 1989; Morris 1990) are also summarily presented.