Proto-planetary nebulae (PPN) are objects in transition between the asymptotic giant branch and planetary nebulae phases. IRAS sources with low (~100-200 K) color temperatures are selected as candidates for identification with mid-infrared photometry. Near-infrared and optical photometry are carried out to determine their spectral energy distributions. PPN often show a double-peak distribution, which is the result of the light from the central star emerging from the dissipating circumstellar dust envelope. Optical spectroscopy is then used to determine the spectral type of the central star. PPN are expected to have spectral types intermediate between AGB stars and planetary nebulae, ranging from G to late A. Many PPN candidates are found to be of luminosity class I, suggesting that the have low surface gravities and extended atmospheres. Molecular spectroscopy at the mm or submm wavelengths are also used to study the chemical composition of their circumstellar envelopes.
A number of proto-planetary nebulae discovered by researchers at the University of Calgary have been imaged by the Hubble Space Telescope.
For a review of this subject, please see Kwok, S. 1993, Annual Reviews of Astronomy and Astrophysics, Vol. 31, p. 69
For a popular account of the discovery of proto-planetary nebulae, please see October 1998 issue of Sky and Telescope magazine