Formation of Interstellar Ices behind Shock Waves

We have used a coupled dynamical and chemical model to examine the chemical changes induced by the passage of an interstellar shock in well-shielded regions. Using this model, we demonstrate that the formation of H 2 O in a shock will be followed in the postshock phase by depletion of the water molecules onto the grain surfaces. To attempt to discriminate between the creation of ices behind shocks and their production by means of grain surface chemistry, we examine the deuterium chemistry of water before, during, and after a shock. We show that chemical evolution in the postshock gas can account for both the deuterium fractionation and the abundance of CO 2 relative to H 2 O observed in interstellar and cometary ices. Given the pervasiveness of shocks and turbulent motions within molecular clouds, the model presented here offers an alternative theory to grain surface chemistry for the creation of ices in the interstellar medium, ices that may ultimately be incorporated into comets.