The Postshock Chemical Lifetimes of Outflow Tracers and a Possible New Mechanism to Produce Water Ice Mantles

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Abstract

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 H 2 O and two molecules which are predicted to have abundances that are signicantly a ected in shock-O 2 , 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 e ects of stochastic shock activity on molecular gas over a cloud lifetime. For both models we separately examine the pure gas-phase chemistry as well as the chemistry including the interactions of molecules with grain surfaces. In agreement with previous studies, we nd that shock velocities in excess of 10 km s~1 are required to convert all of the oxygen not locked in CO into before the gas has an opportunity to cool. For pure gas-H 2 O phase models the lifetime of the high water abundances, or legacy, in the postshock gas is "" H 2 O D(47) ] 105 yr, independent of the gas density. A density dependence for the lifetime of is found H 2 O in gas-grain models as the water molecules deplete onto grains at the depletion timescale.

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