Introduction

Over the last decade, we have come to realize that mass loss on the asymptotic giant branch (AGB) has major effects on the formation of planetary nebulae (PN), and many observable characteristics (e.g. haloes, molecular envelopes) of PN can be traced back to the circumstellar envelopes of their AGB progenitors (Kwok 1982). The large infrared excesses observed in PN are certainly due to the remnant of the AGB envelopes which have cooled as the result of expansion (Kwok 1990, Zhang & Kwok 1991). The detections of the 9.7 µm silicate and the 11.3 µm SiC features, both commonly observed in AGB stars, provide confirmations to this link between AGB and PN (Aitken & Roche 1982, Zhang & Kwok 1990).

However, the infrared spectra of PN also show features not found in AGB stars. The most prominent are the family of features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm, which are attributed to the PAH molecule (Allamandola et al. 1989). It is clear that these molecules must either be synthesized during the transition from the AGB to the PN phase, or they are produced in the AGB atmosphere but only excited in the PN environment. In either case, it would be useful to study the infrared spectra of young PN and transition objects between AGB and PN (or proto-PN) in order to understand the origin of the PAH features.