Formation of Interstellar Ices behind Shock Waves

We have used a coupled dynamical and chemical model to examine the chemicalchanges induced by the passage of an interstellar shock in well shieldedregions. Using this model we demonstrate that the formation of water in a shockwill be followed in the post--shock phase by depletion of the water moleculesonto the grain surfaces. To attempt to discriminate between the creation ofices behind shocks and their production by means of grain surface chemistry, weexamine the deuterium chemistry of water before, during, and after a shock. Weshow that chemical evolution in the post--shock gas can account for both thedeuterium fractionation and the abundance of carbon dioxide relative to waterobserved in interstellar and cometary ices. Given the pervasiveness of shocksand turbulent motions within molecular clouds, the model presented here offersan alternate theory to grain surface chemistry for the creation of ices in theinterstellar medium, ices that may ultimately be incorporated into comets.Comment: 12 pages (2 figures) using aaspp4.sty; Accepted for publication in ApJ Letter