1999A&A...350L..35K

The out-of-plane bending modes of aromatic C-H bond at 11., 12.1, 12.4, and 13.3µm were detected in the ISO SWS spectra of the proto-planetary nebulae IRAS 07134+1005 and IRAS 22574+6609. In addition, unidentified emission features at 17.8, 18.9, and 19.1µm were detected in the SWS01 and SWS06 spectra of IRAS 07134+1005. The similar strengths of the 12.1, 12.4, and 13.3µm features relative to the 11.3µm feature, together with the presence of strong aliphatic CH bands at 3.4 and 6.9µm, demonstrate a lower aromaticitiy of the emitting material, compared to those at the later planetary nebulae stage. This suggests a chemical and physical evolution of the carbonaceous dust during the transition between these two phases.