Theory of Interstellar Shocks

The interstellar medium is intermittently disturbed by violent events­ supernova explosions being but one example-which cause large increases in the local pressure. As the result of the pressure increase, the disturbed region will expand. If the pressure increase exceeds a minimum value, a will develop at the leading edge of this expanding disturb­ ance, and the flow in the neighborhood of this front is referred to as a or wave. Besides supernova explosions, interstellar shock waves may be driven by the pressure of photoionized gas, stellar winds, and collisions between fast-moving clumps of interstellar gas. A shock wave is an irreversible, pressure-driven fluid-dynamical dis­ turbance. The irreversible character is due to entropy generation as ordered kinetic energy is dissipated into heat. In neutral gas, the dissipation is due to molecular viscosity in the shock transition (where large velocity gradients and viscous stresses are present). In low-density plasmas, the dissipation may be collisionless, and due to collective motions of the charged particles and the resulting electromagnetic fields. In partially­ ionized gases, the dissipation may sometimes be primarily due to friction