Proto-Planetary Nebulae Are Not the Precursors of Planetary Nebulae With Close Binary Central Stars

Proto-planetary nebulae are considered to be the evolutionary link between AGB stars and planetary nebulae. They possess many of the physical characteristics expected of such a link and provide valuable information about the evolutionary process in these objects. One important characteristic of proto-planetary nebulae is the prevalence of very strong bipolar and point-symmetric morphologies. In planetary nebulae, these shapes are widely considered to be the result of shaping by a binary companion. Indeed, a number of studies have shown links between strong axial or point-symmetry in planetary nebulae and close binary central star systems that have gone through a common envelope phase. Here, we explore whether the observed proto-planetary nebulae may be the progenitors of planetary nebulae with close binary central stars. We find that despite the strong shaping of proto-planetary nebulae this is unlikely to be the case. We also discuss options for evolution to close binaries and the alternatives.