What Drives Cosmic Chemistry?

It is a relatively straightforward matter to use freely available computer codes and lists of chemical reactions to compute abundances of molecular species for many types of interstellar or circumstellar region. For example, the UDfA, Ohio, and KIDA websites (see Chapter 9) provide lists of relevant chemical reactions and reaction rate data. Codes to integrate time-dependent chemical rate equations incorporating these data are widely available and provide as outputs the chemical abundances as functions of time. For many circumstances, the codes are fast, and the reaction rate data (from laboratory experiments and from theory) have been assessed for accuracy. The required input data define the relevant physical conditions for the region to be investigated.

These codes and databases are immensely useful achievements that are based on decades of research. However, the results from this approach do not readily provide the insight that addresses some of the questions we posed in Chapter 1: What are the useful molecular tracers for observers to use, and how do these tracers respond to changes in the ‘drivers’ of the chemistry? Observers do not need to understand all the details of the chemical networks (which may contain thousands of reactions), but it is important to appreciate how the choice of the tracer molecule may be guided by, and depend on, the physical conditions in the regions they wish to study.