O-bearing molecules in carbon-rich proto-planetary objects: Study of CRL 618

We present ISO LWS observations of the proto-planetary nebula CRL 618, a star evolving very fast to the planetary nebula stage. In addition to the lines of (CO)-C-12, (CO)-C-13, HCN and HNC, we report on the detection of H2O and OH emission together with the fine structure lines of [OI] at 63 and 145 mum. We suggest that O-bearing species other than CO are produced in the innermost region of the circumstellar envelope. The UV photons from the central star photodissociate most of the molecular species produced in the AGB phase and allow a chemistry dominated by standard ion-neutral reactions. Not only allow these reactions the formation of O-bearing species, but they also modify the abundances of C-rich molecules like HCN and HNC for which we found an abundance ratio of similar or equal to 1, much lower than in AGB stars. The molecular abundances in the different regions of the circumstellar envelope have been derived from radiative transfer models and our knowledge of its physical structure. Furthermore, 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.