ANATOMY OF THE PHOTODISSOCIATION REGION IN THE ORION BAR

Much of the interstellar gas resides in photodissociation regions whos e chemistry and energy balance is controlled by the flux of far-ultrav iolet radiation upon them. These photons can ionize and dissociate mol ecules and heat the gas through the photoelectric effect working on du st grains. These regions have been extensively modeled theoretically, but detailed observational studies are few. Mapping of the prominent O rion Bar photodissociation region at wavelengths corresponding to the carbon-hydrogen stretching mode of polycyclic aromatic hydrocarbons, t he 1-0 S(1) line of molecular hydrogen, and the J = 1-0 rotational lin e of carbon monoxide allows the penetration of the far-ultraviolet rad iation into the cloud to be traced. The results strongly support the t heoretical models and show conclusively that the incident far-ultravio let radiation field, not shocks as has sometimes been proposed, is res ponsible for the emission in the Orion Bar.