Ground-Based and IRAS Observations of Proto-Planetary Nebulae

Previous infrared sky surveys (IRC and AFGL) have revealed many asymptotic-giant-branch (AGB) stars which are totally obscured by circumstellar dust. These stars are characterized by the low colour temperatures (200-600K) and molecular emissions (in CO or OH) from the circumstellar envelopes. Subsequent infrared and millimeter observations suggest that they have mass loss rates of 10-5 – 10-4 M⊙ yr -1 and have circumstellar envelopes which are optically thick as far as λ ~ 10 µm. Since the mass loss rates are suspected to increase as stars ascend the AGB, very late AGB stars with low colour temperatures can be discovered by the IRAS sky survey. These stars will soon evolve into planetary nebulae (PN), and therefore the IRAS results may provide valuable information on the evolutionary nhase between AGB and PN.