Traveling self-sustained structures in interstellar clouds with the isentropic instability

The isentropic thermal instability of media with a generalized heat-loss function and negative bulk viscosity condition are discussed. We obtain the nonlinear equation taking into account the nonlinear saturation of the isentropic instability. This equation describes the nonstationary evolution of acoustical waves in media with the isentropic instability. Its stationary solutions are investigated analytically. The most interesting solution is the self-sustained pulse. Using the numerical simulation of the nonlinear acoustical equation and the full system of one-dimensional non-stationary hydrodynamical equations, we showed the disintegration of the initial weak perturbation of compression into sequence of these self-sustained pulses in low-density PDRs.