Protoplanetary nebulae.

Theoretical models of PN origin are discussed, and the results of recent observations of proto-PN are reviewed. The evidence supporting the superwind hypothesis of Renzini (1980) is summarized: in this model the PN precursor (a star of low or intermediate mass at the tip of the AGB) loses mass very rapidly (0.00001 solar mass/yr or faster). Particular attention is given to the transition phase, when the superwind mass loss is complete but the star has not reached the 30,000 K temperature required to initiate ionization of the expanding envelope; data on three candidate transition objects (CRL618, Vy 2-2, and CRL2688) are presented in tables and graphs and briefly characterized.