Photon-Dominated Regions PDRs

The standard approach to modeling PDRs is to use a one-dimensional approach in which the radiation strikes perpendicularly. The region is divided into slabs, so that the equations of radiative transfer and chemical kinetics can be solved conveniently. The slabs can be homogeneous, or can have different gas densities. The radiation is scattered and absorbed by dust particles, but, in addition, both H2 and 

One can view a quiescent molecular cloud as a one-dimensional PDR with % = 1. Here, instead of spherical shells representing outer and inner layers, one has one-dimensional slabs. The advantage of such shell models65 over homogeneous models of the inner portions of clouds is that the roles of the outer layers can be accounted for such roles are especially important for atoms (e.g. C) and radicals (e.g. OH, CH). Small dense clouds, known as translucent clouds, have particularly salient outer portions which should be included in models. 

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Photon-dominated region (PDR) The region around a bright star where photochemical processes dominate the ion-electron, plasma-like state of matter. 

Photodissociation regions (PDRs) are defined as regions where the chemistry is dominated by photons. Hence the chemistry of PDR tracers must be dominated by photo-induced processes, at least indirectly. Reactive species, rapidly destroyed by reactions with H2 or abundant neutrals are therefore good tracers of the illuminated outer layers of molecular clouds. The list includes the gas coolants [CII], [OI] and [Cl], radicals like HCO, CCH or C-C3H2, as well as reactive ions like CO, HOC or CF" ". The rotational and rovibrational lines of H2 are also bright in PDRs. 

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