Shaping Bipolar and Elliptical Planetary Nebulae: Effects of Stellar Rotation, Photoionization Heating, and Magnetic Fields

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Abstract

We present two-dimensional hydrodynamical and magnetohydrodynamical simulations of the evolution of planetary nebulae formed through the interaction of two succeeding, time-independent stellar winds. Both winds are modeled according to a consistent physical prescription for the latitudinal dependence of their properties. We propose that single stars with initial masses above ~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 inflation to a double lobe structure by the post-AGB wind leads to the typical hourglass shape found in many planetary nebulae, such as MyCn 18. Following Chevalier & Luo and Różyczka & Franco, we then combine the effect of a magnetic field in the post-AGB wind with rotating AGB winds. We obtain highly collimated bipolar nebula shapes, reminiscent of M2-9 or He 2-437. For sufficiently strong fields, ansae and jets, similar to those observed in IC 4593 are 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 AGB winds, which we propose for single stars with initial masses below ~1.3 M☉. Photoionization, via instabilities in the ionization-shock front, can generate irregularities in the shape of the simulated nebulae. In particular, it leads to the formation of cometary knots, similar to those seen in the Helix nebula (NGC 7293). This effect may also be responsible for large-scale irregularities like those found in Sh 2-71 or WeSb 4. We arrive at a scenario in which the majority of the planetary nebula with their diverse morphologies is obtained from single stars. This scenario is consistent with the Galactic distribution of the different nebula types, since spherical and elliptical nebulae—which have a distribution with a large scale height above the Galactic plane—are ascribed to progenitor masses below ~1.3 M☉, with magnetic effects introducing ellipticities. Bipolar nebulae, on the other hand—which are on average closer to the Galactic plane—are found to stem from progenitors with initial masses above ~1.3 M☉.

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