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 r egions. Using this model, we demonstrate that the formation of H2O in a sho ck will be followed in tb: postshock phase by depletion of the water molecu les onto the grain surfaces. To attempt to discriminate between the creatio n of ices behind shocks and their production by means of grain surface chem istry, we examine title deuterium chemistry of water before, during, and af ter a shock. We show that chemical evolution in the postshock gas can accou nt for both the deuterium fractionation and the abundance of CO2 relative t o H2O observed in interstellar and cometary ices. Given the pervasiveness o f shocks and turbulent motions within molecular clouds, the model presented here offers an alternative theory to grain surface chemistry for the creat ion of ices in the interstellar medium, ices that may ultimately be incorpo rated into comets.