2 KEY PROCESSES IN DUST GAS CHEMICAL MODELING - PHOTOPROCESSING OF GRAIN MANTLES AND EXPLOSIVE DESORPTION/

Two models of the time dependent chemical evolution of stable dense an d translucent clouds are presented: one for pure gas phase chemistry a nd the other in which solid grain chemistry is included along with the gas. Comparing the results using these two schemes for the theoretica l abundances of certain key molecules shows that including the dust pr ovides a significantly (often by orders of magnitude) better agreement with the observations than those derived by pure gas phase chemistry models. The initial atomic abundances are those given by observations and are not modified to suit the model. Moreover, the inclusion of gra in chemistry appears to minimize the effects of uncertainties in some important gas phase reaction rates, which would otherwise strongly aff ect the results of pure gas phase models. The grain mantle composition and gas phase abundances have been investigated using a number of dif ferent physical assumptions for both dense and translucent cloud model s, taking into consideration the accretion, photochemical processing a nd desorption mechanisms involving the dust grains. The use of trigger ed explosive desorption is critical to providing reasonable steady sta te abundances. The abundances of H2O, H2CO, CH3OH and NH3 have a parti cular relevance because they are more abundantly produced in dust than in the gas. The most abundant observed molecules in grain mantles are H2O and CO which, under irradiation by ultraviolet light, not only pr oduce H2CO but the latter can in turn react with water ice producing C H3OH. The reversible transformation between formaldehyde and methanol in the dust affects their gas phase abundances in both translucent and dense clouds. Depth dependent calculations have been performed and it is found that the effects of solid state photochemical molecular prod uction in the inner part of a dense cloud are much larger than in the outer part or in a translucent cloud. In addition to matching observed gas phase abundances with theory we emphasize the constraint provided by the grain mantle composition.