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We have designed an improved algorithm that enables us to simulate thechemistry of cold dense interstellar clouds with a full gas-grain reactionnetwork. The chemistry is treated by a unified microscopic-macroscopic MonteCarlo approach that includes photon penetration and bulk diffusion. Todetermine the significance of these two processes, we simulate the chemistrywith three different models. In Model 1, we use an exponential treatment tofollow how photons penetrate and photodissociate ice species throughout thegrain mantle. Moreover, the products of photodissociation are allowed todiffuse via bulk diffusion and react within the ice mantle. Model 2 is similarto Model 1 but with a slower bulk diffusion rate. A reference Model 0, whichonly allows photodissociation reactions to occur on the top two layers, is alsosimulated. Photodesorption is assumed to occur from the top two layers in allthree models. We found that the abundances of major stable species in grainmantles do not differ much among these three models, and the results of oursimulation for the abundances of these species agree well with observations.Likewise, the abundances of gas-phase species in the three models do not vary.However, the abundances of radicals in grain mantles can differ by up to twoorders of magnitude depending upon the degree of photon penetration and thebulk diffusion of photodissociation products. We also found that complexmolecules can be formed at temperatures as low as 10 K in all three models.

astrophysical journal/the astrophysical journal

Interstellar Simulations Using A Unified Microscopic-Macroscopic Monte  Carlo Model with a full Gas-Grain Network including Bulk Diffusion in Ice  Mantles