1N4
Sun Kwok 1, Kevin Volk 1, & Bruce J. Hrivnak 2
1 University of Calgary, Canada
2 Valparaiso University, U.S.A.
Proto-planetary nebulae (PPN) are transitions objects between the asymptotic giant branch (AGB) and the planetary nebulae phases of evolution. In the last decade, over 30 PPN have been discovered (cf Kwok 1993, Ann. Rev. Astr. Ap. 31, 63). All show strong infrared excesses due the remnants of the circumstellar envelopes ejected during the AGB phase. For oxygen-rich objects, the 10 and 18 micron silicate features can be seen, confirmiing that they evolve from oxygen-rich AGB stars. For carbon-rich objects, circumstellar molelcular absorption features due to C2 and C3 are often detected in the optical, and emission features at 3.3, 7.7, 11.3 microns due to the PAH molecules are present in the infrared. Most interestingly, strong unidentified emission features at 21 microns (Kwok, Volk, and Hrivnak 1989, ApJ, 345, L51; Kwok et al. 1995, ApJ, 454, 394) and 30 microns (Omont et al. 1995, ApJ, 454, 819) are been detected. The origin of these features are unknown, although they are likely to arise from a large, carbon-based molecule.
With the ISO SWS, we have observed a number of proto-planetary nebulae and obtained high resolution spectra on the 21 and 30 microns features. These results will be presented together with a discussion on the chemical origins of these features.