  Using a radiation-hydrodynamics code I follow the formation of planetary
nebulae around stars of different mass. Because a more massive central star
evolves much faster than a lower mass one, it is to be expected that this will
affect the formation of the PN. For the stars I use the evolutionary tracks for
remnants with masses of 0.605 M0 and 0.836 M0, taken from Bloecker (1995). The
AGB wind is assumed to be concentrated in a thin disk, which in models without
evolving stars leads to the formation of a bipolar nebula. I find that in the
case of the 0.836 M0 remnant the nebula indeed acquires a bipolar shape,
whereas for the 0.605 M0 remnant the shape is more elliptical. The reason for
this is the time it takes to ionize the AGB material; if this happens
sufficiently slowly the density distribution in the AGB wind will be smoothed
out, leading to more elliptical shapes. If it happens quickly, the original
density distribution (in this case a thin disk) is hardly affected. This result
suggests that lower mass central stars will less easily produce bipolar
nebulae, which is supported by observations.
