Unsteady motions of dense envelopes generated by ionization-shock fronts in the interstellar medium

The process of formation and acceleration of a neutral gas envelope during the propagation of an ionization-shock front in a spherical gas cloud is numerically modeled. The velocity and acceleration of the envelope as well as its mass variation with time are determined. The results obtained are compared with some approximate formulas known in the literature. The growth of small velocity perturbations in the dense gas and the effects of mass and momentum accumulation in “finger-like” condensations are studied in the framework of a two-dimensional model.