Chemical modelling of molecular sources

A model is described which is aimed at understanding the chemical processes which occur in cold dark interstellar clouds and is applied to the molecular cloud TMC-1. This source contains the cyanopolyynes and many other complex hydrocarbon species and several of these are included as well as the more simple molecules in the chemical network. The hydrocarbon chemistry is driven by the C+-H2 radiative association reaction and includes C+ insertion reactions, abstraction reactions with H2 and condensation reactions involving CH4 and C2H2. It is shown that the observed abundance of HC3N in TMC-1 requires cyanopolyyne formation to proceed via N atom reactions rather than CN or HCN as has previously been suggested. (author)