1996A&A...312..599F

High angular resolution molecular mapping of the interface between the molecular cloud and the HII region in the Orion nebula has been carried out with the 30-m telescope. The bulk of the molecular emission comes from two molecular bars (Northern and Southern molecular bars) located beyond the ionization fronts of the HII region. Our data show the existence of high density gas, n∼10⁶cm^–3, toward the intense HII region. Very plausibly, this high density gas is forming the back walls of the cavity carved in the cloud by the HII region. In spite of the high hydrogen densities measured in the region, N₂H⁺ and HC₃N have not been detected toward the molecular bars and toward the intense HII region. Our data show that the N₂H⁺ fractional abundance, as well as that of HC₃N, decrease at least a factor of ∼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 (≥10'x10').