New models of interstellar gas-grain chemistry - I. Surface diffusion rates

Abstract

In recent years it has become evident that large differences can exist between model results of grain‐surface chemistry obtained from a rate equation approach and from a Monte Carlo technique. This dichotomy has led to the development of a modified rate equation method, in which a key element is the artificial slowing down of the diffusion rate of surface hydrogen atoms. Recent laboratory research into the surface diffusion rate of atomic hydrogen suggests that atomic hydrogen moves more slowly on grains than heretofore assumed. This research appears to lessen the need for modifications to the rate equation method. Based on the new laboratory work, we have developed appropriate models of gas‐phase and grain‐surface chemistry in quiescent dense cloud cores to examine the chemical effects of slowing down the rate at which atomic H can scan over dust surfaces. Furthermore, we have investigated the effect of slowing down the rate at which all species can move over grain surfaces.

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

“…Finally, the simulations are compared with a steady-state model proposed by Charnley et al (1997) that we have adjusted to take the changing CO gas-phase abundance into account. Figure 8 plots the results for a rate equation model (see, e.g., Ruffle & Herbst 2000) with the same chemical and physical processes as used in our Monte Carlo approach. Besides the difference in mathematical techniques, the rate equation method uses single hopping and desorption rates rather than rates that depend on the local structure.…”

“…Given the large differences in parameters, and assorted methods of calculation among the various studies, large variance in results can be expected. The diamonds show results by Ruffle & Herbst (2000), who used a large reaction network of both gas phase reactions and grain surface reactions. Both chemistries were treated using rate equations for four different scenarios defined by slow and fast diffusion rates combined with atomic and molecular initial conditions.…”

“…The points used here are in the early times of the collapse when the density is still close to the initial density. Again a full network was used, comparable to Ruffle & Herbst (2000), but with intermediate diffusion rates.…”

“…In this manner, a set of energy barriers for the different processes on the surface -diffusion, desorption and reaction -was obtained, which can now be used in an interstellar environment. Unlike the master equation (Stantcheva et al 2002), rate equation (Ruffle & Herbst 2000), and macroscopic Monte Carlo (Ruffle & Herbst 2000) studies of methanol formation, the continuous-time, random-walk (CTRW) Monte Carlo technique accounts for the layering of the CO. This layering is crucial, because the constant addition of fresh CO tends to protect the underlying layers from hydrogenation and limits the time available for reaction.…”

A stochastic approach to grain surface chemical kinetics

“…Charnley (1998) has employed a Monte Carlo technique to solve the master equation governing a model of interstellar gas phase chemistry and first suggested that a master equation approach can be applied to the problems of grain surface chemistry and to the coupling of the gas phase and grain surface chemistries in Send offprint requests to: T. W. Hartquist, e-mail: twh@ast.leads.ac.uk interstellar clouds. If a grain surface is expected to contain few reactive species, such an approach to the treatment of the surface chemistry provides more accurate results than the deterministic rate equation method (Ruffle & Herbst 2000). Owing to the computational expense of Monte Carlo methods, and the fact that, to date, it has not been possible to couple such an approach with a fully time-dependent gas-phase chemistry, Caselli et al (1998) and Shalabiea et al (1998) have introduced and employed modifications to the deterministic rate equations in an attempt to generalize such models so that they can be applied validly to problems in which the surface chemistry is accretion limited.…”