Proto-Planetary Nebulae: Models and IRAS Observations

Abstract

A number of high-galactic latitude supergiant stars of intermediate spectral types have been suggested to be proto-planetary nebulae (PPN), such as the 89 Her and R CrB stars. A number of these stars along with some IRAS sources expected to be PPN -either sources with IRAS low Resolution Spectra (LRS) showing features which may indicate unusually cool dust shells or unusually red IRAS sources for which CO emission from a circumstellar envelope has been observed- where chosen for study. The IRAS 12/25 and 25/60 μm colours of 32 such stars from 3 groups on a colour-colour diagram. Class I show colours similar to ordinary stars; Class II have a 60 μm excess but have normal 12/25 μm colours; Class III are much redder than ordinary stars.