Dual Axisymmetry in Proto-Planetary Nebula Reflection Nebulosities: Results from an HST Snapshot Survey of PPN Candidates

Abstract

We summarize results of our {\it HST} imaging survey of proto-planetary nebula (PPN) candidates, in which we discovered two types of axisymmetric reflection nebulosities. The Star-Obvious Low-level-Elongated (SOLE) nebulae show a bright central star embedded in a smooth, faint extended nebulosity, whereas the DUst-Prominent Longitudinally-EXtended (DUPLEX) nebulae have remarkable bipolar structure with a completely or partially obscured central star. The intrinsic axisymmetry of the PPN reflection nebulosities demonstrates that the axisymmetry often found in planetary nebulae predates the PPN phase. We suggest that the major cause for the apparent dual morphology is the optical depth difference in the circumstellar dust shell rather than the inclination angle effect.