Binary Progenitor Models for Bipolar Planetary Nebulae

We propose an explanation for the positive correlation of bipolar planetary nebulae with massive progenitors in the paradigm of binary system progenitors. We list 10 critical observations, and argue that single-star models for the formation of bipolar planetary nebulae encounter difficulties complying with these observations. On the other hand, binary system progenitors can naturally explain these key observations, and in addition explain the rich varieties of structures possessed by bipolar planetary nebulae. Based on three of the critical observations, and on previous works by Corradi and Schwarz and by Morris, we postulate that the progenitors of bipolar planetary nebulae are binary stellar systems in which the secondary diverts a substantial fraction of the mass lost by the asymptotic giant branch (AGB) primary, but the systems avoid the common envelope phase for a large fraction of the interaction time. The positive correlation of bipolar planetary nebulae with massive progenitors, M ≳ 2 M_☉, is attributed to the larger ratios of red giant branch (RGB) to AGB radii which low-mass stars attain, compared with massive stars. These larger radii on the RGB cause most stellar binary companions, which potentially could have formed bipolar planetary nebulae if the primary had been on the AGB, to interact with low-mass primaries already on the RGB. This scenario predicts that the central stars of most bipolar planetary nebulae are in binary systems having orbital periods in the range of a few days to few times 10 yr.