Modified Models of Gas/Grain Interactions

Abstract

It is customary to use rate equations for grain chemistry in gas-grain chemical models of interstellar clouds (e.g. d’Hendcourt et al. 1985; Hasegawa et al. 1992; Hasegawa & Herbst 1993; Shalabiea & Greenberg 1994). In this technique, the surface reaction rates can be expressed in terms of rate coefficients analogous to those in the gas phase. Work by Tielens (1995; see also Charnley et al. 1997) showed that, at least for a simple system, the rate equation method (hereafter RE) does not reproduce the Monte Carlo (hereafter MC) approach in selected instances for which the latter approach must be regarded as essentially exact. Recently, Caselli et al. (1998) duplicated the work of Tielens (1995) and suggested several modifications to the rate equations, which allow solutions that approach the MC values in the limit when the latter is correct. Caselli et al (1998) confined their attention to rather simple, unrealistic systems. The main aim of this work is to apply their modified rates to a more complex gas-grain model first studied by Hasegawa & Herbst (1993) and to determine if the results are any different from those previously obtained.