Infrared spectra of transition objects and the composition and evolution of carbon dust

We obtained infrared (5-23 μm) spectra of five carbon-rich objects in transition from the asymptotic giant branch (AGB) to the planetary nebula stage of evolution. These spectra show a variety of IR emission features due to circumstellar materials. In particular, all sources show the 6.2 and 7.7 μm bands, commonly observed in PNs and ascribed to polycyclic aromatic hydrocarbon molecules (PAHs). Some transition nebulae also show a strong 6-9 μm plateau characteristic for larger PAH clusters (≃400 C-atoms). A new broad feature at ≃8.8 μm is present in some sources. This feature is distinctly different from the 8.6 μm PAH feature