THE FORMATION OF BIPOLAR PLANETARY-NEBULAE

Using a radiation-hydrodynamics code I follow the formation of planeta ry nebulae around stars of different mass. Because a more massive cent ral star evolves much faster than a lower mass one, it is to be expect ed that this will affect the formation of the PN. For the stars I use the evolutionary tracks for remnants with masses of 0.605 M. and 0.836 M., taken from Blocker (1995). The AGE wind is assumed to be concentr ated in a thin disk, which in models without evolving stars leads to t he formation of a bipolar nebula. I find that in the case of the 0.836 M. remnant the nebula indeed acquires a bipolar shape, whereas for th e 0.605 M. remnant the shape is more elliptical. The reason for this i s the time it takes to ionize the AGE material; if this happens suffic iently slowly the density distribution in the AGE wind will be smoothe d out, leading to more elliptical shapes. If it happens quickly, the o riginal density distribution (in this case a thin disk) is hardly affe cted. This result suggests that lower mass central stars will less eas ily produce bipolar nebulae, which is supported by observations.