Planetary nebulae are spectacular objects in the sky which emit a rich spectrum of recombination lines and collisional-excited lines. Modern space observations have found that planetary nebulae emit strong in almost every part of the electromagnetic spectrum, making them one of the most interesting astrophysical objects to study. Planetary nebulae not only serve as valuable laboratories for atomic processes, they can also be used as standard candles to calibrate the distance scale of the universe, as well as being used as tracers of dark matter in the galactic environment.

Although 95% of stars pass through this stage of evolution before they become white dwarfs, the origin of the planetary nebulae phenomenon was poorly understood as late as the 1970s. It is now believed the planetary nebulae are formed from the remnants of the circumstellar envelopes ejected by their progenitors asymptotic giant branch stars, and are compressed and accelerated into their present ring-like structure by the interacting winds process (Kwok, Purton, & FitzGerald 1978, ApJ, 219, L125).

At the University of Calgary, we have been active in both theoretical and observational studies of planetary nebulae. Models have been constructed to simulate the spectral energy distribution of planetary nebulae by combining dust radiation transfer, ionization models, and stellar evolution models. Over 150 compact PN have been mapped with the VLA at very high (0.4 to 0.1 arc sec) angular resolutions. Many nebulae of high surface brightness (which are therefore likely to be young PN) have been identified. The morphologies seen in these radio images are similar in character and relative frequencies to the optical morphologies, which are sampling a group of much larger and older PN. We seek to understand the general characteristics of the PN morphology and why the shaping of PN appears to happen early in their evolution. We would also like to determine whether the morphology evolves as the nebula ages, despite the similarity of the morphologies of the young PNs in the radio sample to those of the older PN for which optical images are available. The morphologies of PN give us information about the mass loss on the AGB and in the PN phase, for the PN appear to be shaped by the interaction of these two stellar winds.

We are also interested in the nature of the dust emission from these objects. IRAS observations of PN suggest that 30% or more of the total energy output from PN is in the form of dust emission in the infrared. While some PN show the silicate or SiC features, many more display the family of aromatic features in the infrared. How the stars evolve from AGB with primarily silicate and SiC grains to PN with aromatic grains is one of the most fascinating problems of astrochemistry.

For a review of the recent advances in planetary nebulae research, please see: Kwok, S. 2000, The Origin and Evolution of Planetary Nebulae, Cambridge University Press

For a popular account with over 100 images of planetary nebulae, see Kwok, S. 2001, Cosmic Butterflies: The Colorful Mysteries of Planetary Nebulae, Cambridge University Press