ANATOMY OF A PHOTODISSOCIATION REGION - HIGH ANGULAR RESOLUTION IMAGES OF MOLECULAR-EMISSION IN THE ORION BAR

We present observations of the molecular component of the Orion Bar, a prototypical Photodissociation Region (PDR) illuminated by the Trapezium cluster. The high angular resolution (6''-10'') that we have achieved by combining single-dish and interferometric observations has allowed us to examine in detail the spatial and kinematic morphology of this region and to estimate the physical characteristics of the molecular gas it contains. Our observations indicate that this PDR can be essentially described as a homogeneously distributed slab of moderately dense material (similar to 5 x 10(4) cm(-3)), in which are embedded a small number of dense (>10(6) cm(-3)) clumps. The latter play little or no role in determining the thickness and kinetic temperature structure of this PDR. This observational picture is largely supported by PDR model calculations for this region, which we describe in detail in this work. We also find our model predictions of the intensities of a variety of atomic and molecular lines to be in good general agreement with a number of previous observations.