Evolution of carbon-rich proto-planetary objects

We compare ISO LWS observations of three C-rich objects typical of each step of the fast transition of an AGB star to the Planetay Nebula stage: CRL 2688, a very young Proto-Planetary Nebula, CRL 618, a Proto-Planetary Nebula, and NGC 7027, a young Planetary Nebula. Furthermore, we study the min, submm, and IR CO emission from CRL 2688. We underline the violent changes that occur in Clio chemical composition of these objects during their evolution due to the increasing UV radiation field and the strong shocks generated by fast stellar winds. The importance of these mechanisms depends on the degree of evolution of the star. The circumstellar envelopes of evolved stars are important objects for the stellar evolution. As proved by ISO, these objects are very good targets for the Herschel Space Observatory (HSO, ex-FIRST). We present here what the HSO will teach us about these objects.