Molecular Line Emissions from the Photodissociation Region of NGC 7027

We have observed or searched for 11 molecular species in 17 transitions in the 200 and 300 GHz bands in the young planetary nebula NGC 7027. The results include a first detection of C 2 H in this source. The observed spectra of HCO + , H 13 CO + , HCN, CN, C 2 H, and CO + show line widths wider than that of bulk CO emission but coincident with the full width at detection limit of weak wings in CO spectra. The HCO + , HCN, and CN emitting regions have a size of about 13'', significantly smaller than that of the CO emitting region. Thus, the emission of all the observed molecules other than CO and 13 CO must originate from a very small volume compared with the entire CO envelope. Excitation analyses of six molecules with electron and neutral collisions at a gas temperature of 800 K yield similar density estimates ranging from 1.3 to 5 × 10 5 cm -3 . Estimated column densities for HCO + , HCN, CN, CO + , and C 2 H are in good agreement with predictions by a chemical model for the photodissociation region of NGC 7027. These analyses suggest that photochemistry is active in NGC 7027 and the observed molecular emissions (except CO) originate from the H 2 photodissociation zone of the object.