A Monte-Carlo Simulation of the Production of Hydrogen Molecules on Grain Surfaces

It is now confirmed that observed hydrogen molecules in the interstellar medium are formed on the grain surfaces. Dust grains behave like a catalyst for the formation of the hydrogen molecule. After accretion from the gas phase H atoms randomly hopped around the surface to find out the reactant partner. When two H atoms combine a single hydrogen molecule is formed. So far, the average recombination time to produce a hydrogen molecule used to be computed by a crude assumption about the diffusion of the H atom on a grain surface. Here, we perform a Monte-Carlo simulation to exactly quantify the recombination rates for the formation of hydrogen molecule on the grain surface. During our simulation, it is noticed that the production rate is highly dependent upon the grain size and the accretion rate. Since molecular hydrogen is the most abundant species in the ISM, changes in its production rate could affect the other species also. We mainly carried our our simulation for two kind of grains namely, Amorphous Carbon and Olivine because they are very abundant in the ISM.