2001ApJ...558..157S

We discuss the formation of planetary nebulae (PNs) having a pair of lobes, or multilobes, in their inner region, surrounded by an elliptical or spherical shell or halo. Both elliptical and bipolar PNs are considered; when the lobes are much smaller than the main elliptical shell, the PN is termed elliptical, while when the lobes are the main structure of the nebula, the PN is termed bipolar. We suggest that most of these PNs are formed by wide binary systems with final orbital periods in the range of ∼40-10⁴ yr, such that there is no strong tidal interaction. The outer, more spherical structure is formed from the early asymptotic giant branch (AGB) wind. Toward the end of the AGB, the mass-loss rate increases and wind velocity possibly decreases, making the conditions for the formation of an accretion disk around the wide companion more favorable. We assume that once a massive enough accretion disk is formed around the accreting companion, it blows jets or a collimated fast wind, which leads to the formation of a pair of lobes in the inner region. In cases of a precessing accretion disk, a multilobe structure can be formed. We conduct a population synthesis study of such systems and find that overall ∼5%-20% of all PNs are formed by such binary systems. The exact percentage strongly depends on the wind velocities of stars about to leave the AGB. In about half of these systems, the initially more massive star is the AGB star and the accretor is a main-sequence star, while in the other half the initially less massive star is the AGB star and it has a white dwarf accretor. We also estimate that ∼20%-40% of these systems possess observable departure from axisymmetry; e.g., the central star is not in the center of the nebula. Our population synthesis not only supports the binary model for formation of these types of PNs, e.g., Hu 2-1, He 2-113, He 2-47, and M1-37, but more generally supports the binary model for the formation of bipolar and many elliptical PNs.