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 de-excitation of H2. In general the model successfully reproduces the observed H2 line emission from a wide range of energy levels. The observed [SiII]34.8µm emission appears to indicate substantial depletion of Si in NGC 2023.

Introduction

A significant fraction of the ultraviolet radiation emitted by massive stars impinges on the molecular gas associated with star formation. The resulting photodissociation regions (PDRs) therefore play an important role in re-processing the energy flow in star-forming galaxies. Modelling these PDRs is therefore an important theoretical challenge, both to test our understanding of the physical processes in interstellar gas, and to interpret observations of star-forming galaxies.

It is frequently the case that the illuminating star is hot enough to produce an H II region, in which case the photodissociation region is bounded on one side by an ionization front, and on the other by cold molecular gas which has not yet been appreciably affected by ultraviolet radiation. The hv < 13.6eV photons propagating beyond the ionization front raise the fractional ionization, photo-excite and photo-dissociate the H2, and heat the gas via photoemission from dust and collisional de-excitation of vibrationally-excited H2.