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Circumstellar envelopes (CSEs) are rich chemical environments. In an effort to explain observations, many models of these sources have been constructed. Both the chemistry (e. g. Glassgold \& Huggins, 1984; Nejad \& Millar, 1987) and photoprocesses (e. g. Glassgold, et al., 1987; Cherchneff et al, 1993) have been modeled with many levels of detail. Unlike those of molecular clouds, current detailed models of CSEs ignore the effects of grain surface chemistry.
In our case, we consider a superset of the available models. We self-consistently treat the photodissociation of $CO$ and $H_{2}$ through line processes, including both line (through the Sobolev Approximation) and continuum (by exactly solving the transfer equation) radiative transfer. Our chemical network is a combination of the published networks of Cherchneff et al (1993), Glassgold \& Huggins (1984), the Manchester group (1991), and the unpublished networks of Herbst and Leung. We have further enhanced this model by incorporating a pertinent subset of the grain surface chemistry considered by Herbst, Hasegawa, and Leung (1992). This yields a network of 196 species (133 gas phase, and 63 grain surface), and a total of 1260 reactions.
Using this model, we explore the role of grain-surface in circumstellar environments. In particular, we consider the role grains play in the production and hydrogenation of carbonaceous species. We also study the effect of physical conditions on the abundances of grain-surface species. Finally, we compare our results to similar models of quiescent clouds where grain surface chemistry is important in these roles.
