Abstract.A reduced network for CO production and removal, obtained via the use of objective techniques, is shown to produce reliable results over a wide range of physical conditions. Our analysis indicates that reactions involving species with long carbon chains are important in governing the CO abundance in some translucent regions.

Abstract.A model of the grain surface chemistry involving the accretion of atoms of two different elements, X and Y, and their reactions to form species X 2 , XY, and Y 2 was examined for a wide range of choices for the values of its three free parameters -the accretion rate of X and Y, the desorption rate of X and the grain surface sweeping time of Y, all considered relative to the grain surface sweeping rate of X. Relative production rates of the diatomics were calculated with five methods involving, respectively, a high-order truncation of the master equation, a low-order truncation of the master equation, the standard deterministic rate equation approach, a modified rate equation approach and a set of approximations which are in some cases appropriate for accretion dominated chemistry. The accuracies of the relative production rates calculated with the different methods were assessed for the wide range of model parameters. The more accurate of the low-truncation master equation calculations and the standard deterministic rate equation approach gives results which are in most cases within ten or twenty per cent of the results given by the high-truncation master equation calculations. For many cases, the more accurate of the low order truncation and the standard deterministic rate equation approaches is indicated by a consideration of the average number of atoms of the two species on the grain's surface.

Abstract. Recombination onto grain surfaces is important in determining the fractional ionisation and the depletions of elements. The recombination rates of ions depend on the grain charges. We calculate the dependence of the grain charge distribution function on visual extinction for each of a number of cloud models in order to obtain the mean grain charge as a function of visual extinction. We give simple fits to the dependences on grain charge and visual extinction of photoionisation and photoelectric heating rates which may be used in the modelling of photon dominated regions.

Abstract.We have used objective techniques to identify, from a huge chemical network, reduced networks that accurately describe the chemistry governing the fractional ionisation in interstellar molecular clouds under a wide variety of conditions.

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