Models of gas-grain chemistry in dense interstellar clouds with complex organic molecules

The large gas-phase reaction network by Herbst (1989) and Leung (1990) is extended to include grain-surface reactions for a complete model of gas-grain chemistry in dense interstellar clouds. Thermal evaporation is assumed to be the only desorption process, and the role of grains in molecular synthesis are studied by including grain reactions that lead to complex molecules. Coupled differential equations are solved to characterize the time-dependent gas-grain chemistry assuming either steady-state or neutral-atomic initial abundances for the gas phase as well as standard values from pseudo-time-dependent computations. The observed abundances of H2O, CH4, and CO on grain surfaces are examined and found to support the results of the homogeneous gas-grain model for dark interstellar clouds. The standard initial conditions from purely gas-phase models are found to be adequate for describing the gaseous chemistry and surface observations in these interstellar clouds.