INTERSTELLAR CHEMISTRY

In order to interpret spectral observations of interstellar clouds in terms of the physical and chemical conditions which prevail within them, a large body of atomic and molecular data is required. We consider both diffuse clouds, which are permeated by the background ultraviolet radiation field, and dense clouds, whose interiors are shielded from the external ultraviolet radiation by dust scattering and absorption in the outer layers. The categories of chemical reaction which are known or believed to be important are reviewed. The results of chemical models that attempt to simulate conditions in the clouds are summarized. The phenomenon of bistability of the steady-state solutions of the chemical rate equations is considered in detail. Interstellar clouds are perturbed from static equilibrium by shock waves, produced by a variety of mechanisms. The structure of shocks and their influence on the physical and chemical state of the medium are described. Processes intervening in the thermal balance of the medium and the exchange of energy with the environment are discussed.