THE POSTSHOCK CHEMICAL LIFETIMES OF OUTFLOW TRACERS AND A POSSIBLE NEW MECHANISM TO PRODUCE WATER ICE MANTLES

We have used a coupled time-dependent chemical and dynamical model to investigate the lifetime of the chemical legacy in the wake of C-type shocks. We concentrate this study on the chemistry of H2O and O-2, two molecules which are predicted to have abundances that are significant ly affected in shock-heated gas. Two models are presented: (1) a three -stage model of preshock, shocked, and postshock gas; and (2) a Monte Carlo cloud simulation where we explore the effects of stochastic shoc k activity on molecular gas over a cloud lifetime. For both models we separately examine the pure gas-phase chemistry as well as the chemist ry including the interactions of molecules with grain surfaces. In agr eement with previous studies, we find that shock velocities in excess of 10 km s(-1) are required to convert all of the oxygen not locked in CO into H2O before the gas has an opportunity to cool. For pure gasph ase models the lifetime of the high water abundances, or ''H2O legacy, '' in the postshock gas is similar to(4-7) x 10(5) yr, independent of the gas density. A density dependence for the lifetime of H2O is found in gas-grain models as the water molecules deplete onto grains at the depletion timescale. Through the Monte Carlo cloud simulation we demo nstrate that the time-average abundance of H2O, the weighted average o f the amount of time gas spends in preshock, shock, and postshock stag es, is a sensitive function of the frequency of shocks. Thus we predic t that the abundance of H2O, and to a lesser extent O-2, can be used t o trace the history of shock activity in molecular gas. We use previou s large-scale surveys of molecular outflows to constrain the frequency of 10 km s(-1) shocks in regions with varying star formation properti es and discuss the observations required to test these results. We dis cuss the postshock lifetimes for other possible outflow tracers (e.g., SiO and CH3OH) and show that the differences between the lifetimes fo r various tracers can produce potentially observable chemical variatio ns between younger and older outflows. For gas-grain models we find th at the abundance of water-ice on grain surfaces can be quite large and is comparable to that observed in molecular clouds. This offers a pos sible alternative method to create water mantles without resorting to grain surface chemistry: gas heating and chemical modification due to a C-type shock and subsequent depletion of the gas-phase species onto grain mantles.