Nebular properties of proto-planetary nebulae

Recent ground-based observations of cool IRAS sources have led to the discovery of many candidates for protoplanetary nebulae (PPN). These objects have cool dust shells and molecular envelopes reminiscent of the circumstellar envelopes of asymptotic giant branch (AGB) stars. Observations of PPN confirm that the circumstellar envelope ejected during the AGB phase dominates the infrared continuum of post-AGB objects. It is suggested that an infrared sequence can be traced throughout the evolutionary phases from AGB to planetary nebulae.