Introduction

Shock waves are ubiquitous in the interstellar medium (ISM) because efficient radiative cooling allows interstellar gas to cool to temperatures low enough that the sound speed is small compared to the velocities of disturbances in the ISM, such as cloud–cloud collisions, bipolar outflows, expanding HII regions, and supernova explosions. Shock waves in dense molecular gas are almost always radiative: The relative kinetic energy of the shocked and unshocked gas is converted into radiation, and since the radiating gas is dense, it is very bright. Because much of the mass in molecular clouds is obscured by dust, the emission from shocks provides a powerful probe of energetic activity occurring in these clouds. In particular, stars inject large amounts of energy into their surroundings in the process of formation, giving rise to bipolar outflows with velocities in excess of 100 km s−1, characteristic of stellar escape velocities (Lada 1985). Intense maser emission in the 1.35 cm line of water is also observed to be associated with newly formed stars, particularly massive stars, with velocities of tens to hundreds of kilometers per second (Genzel 1986). Understanding the structure and spectrum of the shocks associated with these high velocity flows in dense molecular gas is thus a prerequisite for unraveling the complex physical processes attending the birth of stars.

Early studies of shocks in molecular clouds assumed that the neutrals and ions were tied together into a single fluid, and that the shock front was an abrupt transition on the scale of the molecular mean free path (e.g., Field et al. (1968), Hollenbach and McKee (1979)).