Ionization front in interstellar gas: The structure of ionization fronts

Abstract

A complete analysis of the structures of all types of ionization front is given, together with computed examples. It is shown that the solutions given by Goldsworthy for the propagation of ionization fronts, considered as discontinuities, can be made unique, and the unique solutions are given for the case of cylindrical symmetry. Ionization fronts of all types are shown to be possible, depending on the density of the ionized gas and the spectral type of the radiation. In particular strong D -type and weak R -type ionization fronts (corresponding to strong deflagrations and weak detonations, respectively, in combustion theory) prove to be of importance. The existence of these discontinuities conflicts with the Chapman-Jouguet hypothesis and the reasons for this behaviour are examined in detail. It is concluded that the most important condition for waves of this type to occur is that strong cooling effects should be present, which allow the stagnation enthalpy of the flow to have a maximum before decreasing to an equilibrium value at the rear of the wave. It is suggested that this may be of significance in the theory of other gas-dynamic discontinuities, and in determining the validity of the Chapman—Jouguet hypothesis in general.