Theoretical Models of Photodissociation Fronts

Observations of H2 line emission have revealed higher-than-expected gas temperatures in a number of photodissociation fronts. We discuss the heating and cooling processes in photodissociation regions. Observations of NGC 2023 are compared to a theoretical model in which there is substantial gas at temperatures T = 500-1000K heated by photoelectric emission and collisional deexcitation of H2. In general the model successfully reproduces the observed H2 line emission from a wide range of energy levels. The observed [SiII]34.8um emission appears to indicate substantial depletion of Si in NGC 2023.