Numerical simulation of unstable two-dimensional motions of a circumstellar shell

The growth of two-dimensional axisymmetric perturbations in the motion of a neutral shell formed in the interstellar medium when an ionization-shock front exits at the surface of a cloud is simulated numerically. The perturbations are assumed to emerge when the shock ahead of the ionization front reaches the cloud boundary. For long-wavelength perturbations, the accumulation of mass has been found to take place in radially oriented condensations in the shape of a rod pointed toward the star and widened at the opposite end as a result of instability. The shell fragmentation is accompanied by supersonic spouting of a hot plasma into a low-density medium. Flow nonstationarity is shown to affect significantly the gas density and velocity distributions both inside and in the immediate vicinity of the condensation. As one recedes from the ionization front, the density of charged particles changes only slightly, which is inconsistent with the power law of density decrease with increasing distance from the condensation center commonly used in interpreting observations.