Bipolar Preplanetary Nebulae: Hydrodynamics of Dusty Winds in Binary Systems. I. Formation of Accretion Disks

Abstract

Mass-losing giant stars and objects in transition to planetary nebulae often display a bipolar symmetry in their outflowing circumstellar envelopes. Here we initiate a detailed study of the hypothesis that the observational characteristics of asymmetric and bipolar preplanetary nebulae result from the effects of a detached binary companion upon the otherwise spherical wind of a single mass-losing star. We follow the gas flows in this system using three-dimensional smoothed particle hydrodynamics models. In this first work we describe the dusty wind models and the numerical procedure employed, and we address the issue of wind accretion and the formation of accretion disks about the binary companion to the mass-losing giant as a function of wind velocity and binary separation. In all of our models, we find that permanent, stable accretion disks of various sizes form around the binary companion. The disks are geometrically thin, and their equilibrium structure has elliptical streamlines with a range of eccentricities. Our results also indicate that such disks may be susceptible to tilt or warping instabilities. We also find that wind accretion in such binaries is stable, displaying no evidence for any type of flip-flop instability.