Post-AGB Objects as a Phase of Stellar Evolution

The term proto-planetary nebulae (PPNe), in the context of the late stages of stellar evolution, was created just over 20 years ago, to express the belief that in the near future these objects will become planetary nebulae (PNe). The first proto-planetary nebulae (also called post-Asymptotic Giant Branch or post-AGB objects) were discovered in the mid-1970s in the course of the Air Force Sky Survey. Investigation of this phase of stellar evolution developed very rapidly in the 1980s after the IRAS mission when it became clear that proto-planetary nebulae emit a significant part of their energy in the mid- and far-infrared. A new impetus in this field began in the 1990s with high spatial resolution imaging in mid-infrared and optical wavelengths. Evidence has been found that proto-planetary nebulae (at least their central parts) are quite asymmetric while shells of AGB stars display spherical shapes. The most intriguing challenge now is to understand which physical processes are dominant during formation and evolution of proto-planetary nebulae. Is it magnetic field, evolution in binary systems, planets, axisymmetric superwind mass loss and its further shaping by fast wind, collimated jets interacting with a spherically symmetric AGB remnant or maybe an interplay between these different processes? Another challenge is to explain why the mass loss process near the end of the AGB evolution is modulated on timescales of a few hundred years. The model presented during this workshop seems to be very promising in this respect.