Chemical evolution of carbonaceous materials in the last stages of stellarevolution

The out-of-plane bending modes of aromatic C-H bond at 11.3, 12.1, 12.4, an d 13.3 mu m were detected in the ISO SWS spectra of the proto-planetary neb ulae IRAS 07134+1005 and IRAS 22574+6609. In addition, unidentified emissio n features at 17.8, 18.9, and 19.1 mu m were detected in the SWS01 and SWS0 6 spectra of IRAS 07134+1005. The similar strengths of the 12.1, 12.4, and 13.3 mu m features relative to the 11.3 mu m feature, together with the pre sence of strong aliphatic CH bands at 3.4 and 6.9 mu m, demonstrate a lower aromaticitiy of the emitting material, compared to those at the later plan etary nebulae stage. This suggests a chemical and physical evolution of the carbonaceous dust during the transition between these two phases.