The formation of bipolar planetary nebulae

Abstract

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.