Introduction

Conditions of thermodynamic equilibrium are the exception, rather than the rule, in the interstellar medium. In order to interpret the observed intensities of molecular emission lines, it is usually necessary to know the relevant collisional and radiative transition rates. If the lines are optically thin, they do not undergo significant reabsorption within the region emitting the radiation, and the emitted flux is obtained as the line-of-sight integral of the rate of emission per unit volume of gas. However, it is often the case that strong emission lines are optically thick or, at least, have a significant optical depth (i.e. an optical depth of the order of 1) at their centres. Under these circumstances, it is necessary to solve the equation of radiative transfer in order to predict the emitted line fluxes to a reasonable degree of accuracy.

Solving radiative line transfer problems is no mean task. Both analytical and stochastic (Monte-Carlo) approaches are followed, with the latter being more readily applicable when the geometry or the density distribution does not admit simple treatments; this is likely to be always the case of interstellar molecular clouds. Unfortunately, it is also the case that the geometry and the density distribution are generally poorly known or unknown.