MODELS OF CLUMPY PHOTODISSOCIATION REGIONS

We investigate far-infrared (FIR) fine structure line and submillimeter line emissions important in the cooling of photodissociation regions (PDRs) as they would arise in clumpy regions. We have developed a simple model for clumpy PDRs consisting of two components, dense clumps (5 x 10(5), 10(6), or 10(7) cm-3) embedded in a less dense interclump medium (300 or 3000 cm-3). Both the clump and interclump gas is presumed to be heated, ionized, and dissociated by penetrating far-ultraviolet (FUV) (6 < hv < 13.6 eV) photons. Employing the two-component medium radiative transfer formalism of Boisse, we calculate the UV penetration into each model clumpy region. As previously noted by Boisse, the UV radiation penetrates further into the clumpy region (an order of magnitude or more in our cases) than a cloud in which the material is homogeneously distributed. Hence, PDR cooling line emission can be excited on these larger scales. With clump to interclump density contrasts of more than 100, the penetration depends primarily on the density of the interclump medium and less on the clump filling factor. The homogeneous PDR models of Tielens and Hollenbach are used to calculate the line intensities arising in the clumps and in the interclump medium which are assumed to be independent in velocity space. The line intensities are presented for two geometries: face-on and edge-on. In reality, molecular regions have a range of densities and density gradients. Our model incorporates only some of this complexity as a first attempt to study the effects of clumpiness on the resulting line emission and hence the reader should view the results in the paper as investigative. In light of this study, we review observational evidence for clumpy PDRs. In particular, the dense clumps present in this model are capable of reproducing the very high excitation lines (e.g., CO 14-13) and the lower density interclump gas produces the very extended (1-6 pc) [C I] and [C II] emission. In addition, the model suggests an explanation for the unusually high [Si II] (35 mum) intensity with respect to the [O I] (63 mum) intensity found in some PDRs. Most important, the model infers that some lines arise mostly from the interclump material ([C I], [C II], and lower CO transitions) while others are exclusively excited in the clumps ([O I], [Si II], and high CO transitions). This result differentiates our model from previous clumpy models of PDRs which assume that all emission arises from the clumps.