Millimeter and submillimeter observations of the Orion Bar. I. Physical structure.

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 C^18^O, H_2_CO, 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_H_2__=~1^ +3.0^_-0.7_10^6^cm^-3^ and ~90% in a homogeneous interclump medium of n_H_2__=~3^+2.0^_-2.2_10^4^cm^-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.