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 toinvestigate the lifetime of the chemical legacy left in the wake of C-typeshocks. We concentrate this study on the chemistry of H2O and O2, two moleculeswhich are predicted to have abundances that are significantly affected inshock-heated gas. Two models are presented: (1) a three-stage model ofpre-shock, shocked, and post-shock gas; and (2) a Monte-Carlo cloud simulationwhere we explore the effects of stochastic shock activity on molecular gas overa cloud lifetime. In agreement with previous studies, we find that shockvelocities in excess of 10 km s^-1 are required to convert all of the oxygennot locked in CO into H2O before the gas has an opportunity to cool. For puregas-phase models the lifetime of the high water abundances, or ``H2O legacy'',in the post-shock gas is 4 - 7 x 10^5 years. Through the Monte Carlo cloudsimulation we demonstrate that the time-average abundance of H2O is a sensitivefunction of the frequency of shocks. Thus we predict that the abundance of H2Oand other known outflow tracers can be used to trace the history of shockactivity in molecular gas. For gas-grain models we find that the abundance ofwater-ice on grain surfaces can be quite large and is comparable to thatobserved in molecular clouds. This offers a possible alternative method tocreate water mantles without resorting to grain surface chemistry: gas heatingand chemical modification due to a C-type shock and subsequent depletion of thegas-phase species onto grain mantles.Comment: 31 pages (including 16 figures), using aas2pp4.sty. To be published in ApJ, June 1 1998 issu