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

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Abundance - Infrared: Stars - Molecules -, Line: Identifications - Planetary Nebulae: Crl618 -, Stars: Carbon

Scientific paper

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 COd, COt, HCN and HNC, we report on the detection of water and OH emission together with the fine structure lines of [OI] at 63 and 145 μm. The abundance of the later three species relative to COd are 4 10-2, 8 10-4 and 4.5 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 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 ≃ 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.

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