Models for the structure and origin of bipolar nebulae

Consideration of structural models for bipolar nebulae leads to an investigation of how such a geometry may arise. Although nonradial pulsation, rotationally forced mass ejection by a single star, and mass loss from a common envelope binary are discussed, emphasis is put on a bipolar nebulae origin from a binary star system in which the primary is evolving up the red giant branch to the point at which its radius approaches its tidal radius. Numerical models of this phenomenon show that gravitational ejection from an asynchronous binary system leads to terminal outflow velocities in the observed 20-50 km/sec range, and that the rate of mass loss and the time scale over which the mass ejection takes place are consistent with observations if the particle density in the outer layers of the primary star's atmosphere, from which the material is extracted, is in the range of 10 to the 14th to 10 to the 15th/cu cm.