On the origin of infrared plateau features in proto-planetary nebulae

The emission profiles of the 8 and 12 mum plateau features are derived from the Infrared Space Observatory, spectra of proto-planetary nebulae. We suggest that these plateau features are primarily the result of alkane and alkene side groups on very large aromatic molecules and small carbonaceous particles. The relationship between the narrow aromatic hydrocarbon features and the 8 and 12 pm plateau features is discussed in a model of circumstellar chemical evolution where photochemical processing transforms the more aliphatic material in proto-planetary nebulae into more aromatic matter in planetary nebulae.