O-bearing Molecules in Carbon-rich Proto–Planetary Objects

We present Infrared Space Observatory long-wavelength spectrometer observations of the proto-planetary nebula CRL 618, a star evolving very fast toward the planetary nebula stage. In addition to the lines of 12CO, 13CO, HCN, and HNC, we report on the detection of H2O and OH emission together with the fine-structure lines of [O i] at 63 and 145 µm. The abundance of the latter three species relative to 12CO are 4x10-2, 8x10-4, and 4.5 (approximate value) in the regions where they are produced. 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 asymptotic giant branch (AGB) phase and allow a chemistry dominated by standard ion-neutral reactions. They not only allow these reactions for the formation of O-bearing species but also modify the abundances of C-rich molecules like HCN and HNC for which we found an abundance ratio of approximately 1, which is 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 from our knowledge of its physical structure.