Advancing H2 dissociation fronts.

Previous studies of the influence of UV radiation on molecular hydrogen clouds have assumed steady state conditions. Time-dependent effects are investigated which occur when a hot star, surrounded by a dense cloud, begins its main-sequence phase. It is found that an H2 dissociation front will propagate out from the star and heat the gas. The characteristics of the gas flow are discussed by exploiting the similarity to the well-studied theory of ionization fronts and expanding H II regions. A detailed theory is presented for the structure of a plane-parallel dissociation front, and the resulting column densities of excited molecules and warm H I which may be observed are predicted.