Why Every Bipolar Planetary Nebula is `Unique'

Abstract

We present the many evolutionary routes that progenitors of bipolar planetary nebulae (BPNe) can take. Overall, there are about a hundred qualitatively different evolutionary routes, hence about a hundred qualitatively different types of BPNe. Within each type there are quantitative differences as well. Adding the dependence of the appearance on inclination, we find that the number of different apparent structures of BPN is about equal to, or even larger than the number of known BPNe and proto-BPNe. Accordingly we argue that every BPN is a `unique' object in its appearance, but all can be explained within the binary model paradigm. Therefore, we request a stop to the attaching of adjectives such as `unique', `peculiar', and `unusual' to BPNe and proto-BPNe, thereby removing the need to invoke a new model for almost every `unusual' BPN. As a case study we try to build a binary model for the proto-BPN OH231.8+4.2. In our preferred model the AGB Mira-type star has a main sequence companion of mass 1 Mo, orbital period of five years, and eccentricity of 0.1.