Instability of a thin photoevaporable circumstellar envelope

We investigate the stability of a dense neutral shell that is accelerated outward by the hot-gas pressure and that loses its mass through photoionization by radiation from the central star. We assume the H I shell to be thin and use the Lagrangian coordinates to describe its motion. We show that a flow accompanied by cumulative effects emerges during the nonlinear development of the instability. We estimate the influence of the radiative cooling rate on the motion and determine parameters of the gas in the cumulative region. The results obtained are compared with the observations of the nebulae NGC 7293 and NGC 2392.