Interstellar Simulations Using A Unified Microscopic-Macroscopic Monte
  Carlo Model with a full Gas-Grain Network including Bulk Diffusion in Ice
  Mantles
  We have designed an improved algorithm that enables us to simulate the
chemistry of cold dense interstellar clouds with a full gas-grain reaction
network. The chemistry is treated by a unified microscopic-macroscopic Monte
Carlo approach that includes photon penetration and bulk diffusion. To
determine the significance of these two processes, we simulate the chemistry
with three different models. In Model 1, we use an exponential treatment to
follow how photons penetrate and photodissociate ice species throughout the
grain mantle. Moreover, the products of photodissociation are allowed to
diffuse via bulk diffusion and react within the ice mantle. Model 2 is similar
to Model 1 but with a slower bulk diffusion rate. A reference Model 0, which
only allows photodissociation reactions to occur on the top two layers, is also
simulated. Photodesorption is assumed to occur from the top two layers in all
three models. We found that the abundances of major stable species in grain
mantles do not differ much among these three models, and the results of our
simulation 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 two
orders of magnitude depending upon the degree of photon penetration and the
bulk diffusion of photodissociation products. We also found that complex
molecules can be formed at temperatures as low as 10 K in all three models.
