Introduction

Shock waves are common phenomena in the interstellar medium. Shocks occur whenever material moves at velocities exceeding the sound velocity in the surrounding medium and the upstream material cannot dynamically respond to the upcoming material until it arrives. The shock will then compress, heat, and accelerate the medium. The heated material cools through the emission of line photons, further compressing the medium.

In the interstellar medium, two types of shocks are of interest. First, for fast shocks, the gas is so suddenly stopped and heated to a high temperature that insignificant radiative and non-radiative relaxation can take place; e.g., the shock front is much thinner than the postshock relaxation layer. As a result, we can cast the mass, momentum, and energy conservation equations in terms of simple “jump” conditions that relate the preshock (front) and postshock (front) density, temperature, and velocity to each other. These shocks are called J-shocks (J for jump). Second, for weak shocks in a magnetized medium with a low degree of ionization, the shock work is done by trace ions drifting through the predominantly neutral medium. In such shocks, the shock front is much thicker than the cooling length scale and the temperature is set by the balance between heating and cooling. In this case, we cannot use jump conditions but, rather, have to solve the conservation equations for the shock front structure. These shocks are called C-shocks (C for continuous).