Introduction

Interstellar chemistry began to be studied in a fairly serious way when, in the late 1960s and early 1970s, it was demonstrated that a wide variety of molecules existed in dense molecular clouds. At first the main effort was in identifying the main chemical routes by which molecules were formed and destroyed. It was realized that, even in dark molecular clouds where starlight is excluded, cosmic rays may penetrate and cause ionizations which drive a chemistry which would otherwise ‘run down’. This chemistry would, therefore, be largely one of positiveions and molecules. This early recognition met with great success and – although the level of ionization in molecular clouds remains uncertain – the detection of interstellar ions such as HCO+ and N2H+ is strong support for positive ionneutral molecule chemistry. Models of interstellar chemistry involving hundreds or even thousands of reactions are now routinely studied: some of these reactions may be important.

These early studies, understandably, concentrated on the chemistry. They deliberately made the dynamics as simple as possible. Thus, uniform density and temperature were usually invoked, in geometrically convenient shapes such as semi-infinite slabs, or spheres. Steady-state calculations were often performed, without a full consideration of the applicability of steady-state. Later studies showed that it might take around 30 million years to achieve steady-state in molecular clouds, and it was realised that such extended periods might not be available in interstellar clouds.

At the same time, detailed observations were indicating that molecular clouds were far from simple objects.