1995A&A...297..567T

We present observations of the ³P₁-³P₀ fine structure transition of atomic carbon, as well as of the J=2-1 transition of ¹³CO and C¹⁸O toward the Orion Bar, the south-eastern rim of the cavity carved out in the Orion molecular cloud by the HII region surrounding the Trapezium stars. The Orion Bar is a dense region illuminated by an intense flux of UV photons, i.e. a photodissociation or photon-dominated region (PDR). The observed Ci intensity agrees with models of uniform-density edge-on PDRs, which predict a spatially stratified morphology for the ionized, neutral, and molecular components. Our observations, however, show a surprisingly good spatial correlation between Ci and ¹³CO. The bright neutral carbon and ¹³CO emission from the Bar is superposed on a broad plateau of extended emission. The origin of this extended distribution may lie in the particular geometry of the region, which gives rise to face-on PDRs associated with the back and front walls of the cavity. Because of their weak emission, we infer that these face-on PDRs must be clumpy with a low beam filling factor (∼30-40%) in contrast to the Bar which has been shown to have a filling factor of ∼1 for radiation from the ionizing stars.