Advanced stages of stellar evolution, stellar winds, planetary nebulae, interstellar chemistry, infrared spectroscopy

Dr. Kwok's main research interest has been on the effects of mass loss on the late stages of stellar evolution. He has proposed that radiation pressure on grains is the mechanism of mass loss in asymptotic giant branch (AGB) stars (Kwok 1975, ApJ, 198, 583), and showed that OH maser emission from AGB stars is a manifestation of mass loss (Kwok 1976 JRASC, 70, 49). Through infrared and millimetre-wave spectroscopic observations, he found that the mass loss rates of AGB stars are so high that they should have significant effects on AGB evolution (Kwok 1987, Physics Reports,156,3). He also promoted the idea that the AGB is terminated by the complete removal of the hydrogen envelope through mass loss, before carbon can be ignited. The exposure of the core and the subsequent initiation of another fast wind, lead to a "snow-plow" effect that creates a planetary nebula (Kwok, Purton, & FitzGerald 1978, ApJ, 219, L125). This "interacting-winds" theory has become the standard model of planetary nebulae formation, and has led to a new understanding of the dynamical evolution of planetary nebulae as well as the origin of their different morphologies (Kwok 1994, PASP,106, 1).

After the IRAS mission, Kwok and Bruce Hrivnak began to search for "proto-planetary nebulae (PPN)", the missing link between the AGB and planetary nebula phases using infrared colors as criteria. Over 30 candidates of PPN were discovered (Kwok 1993, Ann. Rev. Astron. Astrophys. 31, 63).  Follow-up observations by the HST have shown that many of these PPNs have bipolar morphology, suggesting that the shaping of PNs occurs soon after the AGB.

The importance of PPNs goes beyond the shaping of PNs.  Active chemical synthesis is ongoing in the circumstellar envelopes of PPNs.  The unidentified 21 mircon emission feature, believed to originate from an carbonaceous compound, is discovered in PPNs ( Kwok, Volk, & Hrivnak, 1989, ApJ, 345, L51).   ISO observations of PPNs have shown that complex organic molecules are being synthesized, including aromatic compounds with aliphatic side groups (Kwok, Volk, & Hrivnak, 1999, Astr. Ap., 350, L35; Kwok, Volk, & Bernath, 2001, ApJ, 554, L87).   We now recognize that the circumstellar synthesis of inorganic and organic matter plays an important role in the chemical enrichment of the Galaxy and the early solar system  (Kwok 2004, Nature, 430, 985)

Dr. Kwok is leading the Canadian effort in the international Odin mission, which was successfully launched in 2001 and is expected to operate to 2005.  Major results from Odin include the first spectral scan of Orion KL in the submm region not accessible from the ground. 

His book The Origin and Evolution of Planetary Nebulae was published by the Cambridge University Press in 2000.  A popular book on planetary nebulae, Cosmic Butterflies, was published by Cambridge University Press in 2001.  These two books can be ordered from the Cambridge University Press or Amazon.com.  His latest book Physics and Chemistry of the Interstellar Medium will be published by University Science Books in 2005.