1995A&A...294..792H

Millimeter and submillimeter observations of 53 rotational transitions of 18 different molecules and their isotopic varieties are presented at five positions on a strip crossing the Orion Bar. These data are used to study the physical and chemical conditions with increasing depth into the photon-dominated region (PDR). It is found that all molecules show about the same intensity distribution across the Bar. They peak ∼30'' outside the ionization front, and fall off to about 10-20% of their peak values on either side. An excitation analysis of C¹⁸O, H₂CO, CS and HCO⁺ shows that the Orion Bar is best described by an inhomogeneous density distribution where ∼10% of the molecular material is comprised in clumps of hydrogen number density n_H2≃1^ +3.0-0.7_10⁶cm^–3 and ∼90% in a homogeneous interclump medium of n_H2≃3^+2.0_–2.210⁴cm^–3. The kinetic temperature is found to be 85±30K. No significant variations in these physical parameters occur across the Bar. It is therefore concluded that the observed peak in the molecular rotational emission corresponds to a peak in molecular column densities. A geometric model for the Orion Bar is proposed in which with increasing distance from the ionizing stars, the PDR changes from a face-on to an edge-on geometry and back. The increased emission is then caused by the increased length of the line-of-sight through the PDR. This model is found to correspond well with other observations.