Chemical Evolution of the Circumstellar Envelopes of Carbon-rich Post-Asymptotic Giant Branch Objects

Abstract

We have observed, with the 30 m IRAM telescope, the Caltech Submillimeter Observatory telescope, and the Infrared Space Observatory (ISO) satellite,3 the rotational lines of CO at millimeter, submillimeter, and far-IR wavelengths in the direction of carbon-rich stellar objects at different stages of evolution: CRL 2688 (a very young proto-planetary nebula), CRL 618 (a proto-planetary nebula), and NGC 7027 (a young planetary nebula). Several changes in the longwave emission of CO and other molecules are discussed here in relation with the degree of evolution of the objects. In the early stages, represented by CRL 2688, the longwave emission is dominated by CO lines. In the intermediate stage, e.g., CRL 618, very fast outflows are present which, together with the strong UV field from the central star, dissociate CO. The released atomic oxygen is seen via its atomic lines and allows the formation of new O-bearing species, such as H₂O and OH. The abundance of HNC is enhanced with respect to HCN as a result of the chemical processes occurring in the photodissociation region. At this stage, CO lines and [O I] lines are the dominant coolants, while the cooling effect of [C II] is rising. At the planetary nebula stage, e.g., NGC 7027, large parts of the old CO AGB material have been reprocessed. The spectrum is then dominated by atomic and ionic lines. New species, such as CH⁺, appear. Water has probably been reprocessed in OH.