3-PHASE CHEMICAL-MODELS OF DENSE INTERSTELLAR CLOUDS - GAS, DUST PARTICLE MANTLES AND DUST PARTICLE SURFACES

Gas-grain models of the chemistry of dense interstellar clouds are presented in which two grain phases surrounding refractory cores are included. In addition to the actual grain surfaces, on which chemistry occurs. grain mantles. which are formed as adsorbed material trapped below new layers of such material, are considered. Material in grain mantles is assumed to be chemically inert, and therefore protected against destruction. The dust grain and gas temperatures are fixed at 10 K and the gas density n = n(H) + 2n(H-2) remains at approximately 2 x 10(4) CM-3 in these models. Differences between the current three-phase (gas-mantle-surface) model results and those of previous two-phase (gas-surface) models for the overall grain abundances of individual species are small for inert species, but can be substantial for chemically reactive species, especially at late times (greater-than-or-equal-to 10(6) yr) after non-reactive, protected mantles have been built up. It is possible to obtain molecular abundances for different mantle layers deposited at different times; we find that all layers are dominated by water ice. The calculated condensed phase abundance of CO depends strongly on the surface abundance of atomic hydrogen.