CHEMISTRY IN THE HIGH-DENSITY MOLECULAR INTERFACE SURROUNDING THE ORION NEBULA

High angular resolution molecular mapping of the interface between the molecular cloud and the HII region in the Orion nebula has been carri ed out with the 30-m telescope. The bulk of the molecular emission com es from two molecular bars (Northern and Southern molecular bars) loca ted beyond the ionization fronts of the HII region. Our data show the existence of high density gas, n similar to 10(6) cm(-3), toward the i ntense HII region. Very plausibly, this high density gas is forming th e back walls of the cavity carved in the cloud by the HII region. In s pite of the high hydrogen densities measured in the region, N2H+ and H C3N have not been detected toward the molecular bars and toward the in tense HII region. Our data show that the N2H+ fractional abundance, as well as that of HC3N, decrease at least a factor of similar to 10 in these regions, in agreement with the predictions of photodissociation regions (PDRs) models. The uniformity of the chemical conditions over an extended region suggests that we are dealing with a large PDR or an ensemble of face-on and edge-on PDRs (greater than or equal to 10'x10 ').