Shaping bipolar and elliptical planetary nebulae: Effects of stellar rotation, photoionization heating, and magnetic fields

We present two-dimensional hydrodynamical and magnetohydrodynamical simulat ions of the evolution of planetary nebulae formed through the interaction o f two succeeding, time-independent stellar winds. Both winds are modeled ac cording to a consistent physical prescription for the latitudinal dependenc e of their properties. We propose that single stars with initial masses abo ve similar to 1.3 M. can achieve near-critical rotation rates during their "superwind" phase at the tip of the asymptotic giant branch (AGB). We show that the resulting equatorially confined winds and their subsequent inflati on to a double lobe structure by the post-AGE wind leads to the typical hou rglass shape found in many planetary nebulae, such as MyCn 18. Following Ch evalier & Luo and Rozyczka & France, we then combine the effect of a magnet ic field in the post-AGE wind with rotating AGE winds. We obtain highly col limated bipolar nebula shapes, reminiscent of M2-9 or He 2-437. For suffici ently strong fields, ansae and jets, similar to those observed in IC 4593 a re formed in the polar regions of the nebula. Weaker fields are found to be able to account for the shapes of classical elliptical nebulae, e.g., NGC 6905, in the case of spherically symmetric AGE winds, which we propose for single stars with initial masses below similar to 1.3 M.. Photoionization, via instabilities in the ionization-shock front, can generate irregularitie s in the shape of the simulated nebulae. In particular, it leads to the for mation of cometary knots, similar to those seen in the Helix nebula (NGC 72 93). This effect may also be responsible for large-scale irregularities lik e those found in Sh 2-71 or WeSb 4. We arrive at a scenario in which the ma jority of the planetary nebula with their diverse morphologies is obtained from single stars. This scenario is consistent with the Galactic distributi on of the different nebula types, since spherical and elliptical nebulae-wh ich have a distribution with a large scale height above the Galactic plane- are ascribed to progenitor masses below similar to 1.3 M., with magnetic ef fects introducing ellipticities. Bipolar nebulae, on the other hand-which a re on average closer to the Galactic plane-are found to stem from progenito rs with initial masses above similar to 1.3 M..