The formation and evolution of bright rims

Numerical calculations in plane parallel and spherical symmetry of the interaction of initially supersonic weak R ionization fronts with dense clouds are presented. The high resolution achieved allows to study the flow of ionized gas as it streams away from the clouds. A parameter A = photon flux/(recombination coefficient × initial cloud radius × cloud-density²) is shown to be the scaling quantity that prescribes the evolution of the flow in spherical symmetry. Also, the maximum densities in the flow of ionized gas, i.e. at the ionization front itself, are shown to be a function of A, irrespective of the type of ionization front generated during the evolution. Finally, the authors' models are confronted with observations of bright rims.