GAS-PHASE CHEMISTRY IN DENSE INTERSTELLAR CLOUDS INCLUDING GRAIN SURFACE MOLECULAR DEPLETION AND DESORPTION

We present time-dependent models of the chemical evolution of molecula r clouds which include depletion of atoms and molecules onto grain sur faces and desorption, as well as gas-phase interactions. We have inclu ded three mechanisms to remove species from the grain mantles: thermal evaporation, cosmic-ray-induced heating, and photodesorption. A wide range of parameter space has been explored to examine the abundance of species present both on the grain mantles and in the gas phase as a f unction of both position in the cloud (visual extinction) and of evolu tionary state (time). The dominant mechanism that removes molecules fr om the grain mantles is cosmic-ray desorption. At times greater than t he depletion timescale, the abundances of some simple species agree wi th abundances observed in the cold dark cloud TMC-1. Even though cosmi c-ray desorption preserves the gas-phase chemistry at late times, mole cules do show significant depletions from the gas phase. Examination o f the dependence of depletion as a function of density shows that when the density increases from 10(3) cm-3 to 10(5) cm-3 several species i ncluding HCO+, HCN, and CN show gas-phase abundance reductions of over an order of magnitude. The CO:H2O ratio in the grain mantles for our standard model is on the order of 10:1, in reasonable agreement with o bservations of nonpolar CO ice features in rho Ophiuchus and Serpens. We have also examined the interdependence of CO depletion with the spa ce density of molecular hydrogen and binding energy to the grain surfa ce. We find that the observed depletion of CO in Taurus is inconsisten t with CO bonding in an H2O rich mantle, in agreement with observation s. We suggest that if interstellar grains consist of an outer layer of CO ice, then the binding energies for many species to the grain mantl e may be lower than commonly used, and a significant portion of molecu lar material may be maintained in the gas phase.