Kinetic Monte Carlo method for simulating astrochemical kinetics: Test calculations of molecular hydrogen formation on interstellar dust particles

Abstract

Molecular hydrogen formation on the surfaces of interstellar dust grains has been studied numerically. Different stochastic methods (analogue Monte Carlo methods, direct solutions of the chemical master equation, continuous-time random-walk simulations, etc.) have been recently applied to the astrochemistry of hydrogen formation; however, the computational efficiency of these approaches is usually low because they are extremely time consuming (Herbst and Shematovich, 2003). A kinetic version of the Monte Carlo method based on splitting by physical processes is presented in the study. Each of the basic physical processes—adsorption of atomic hydrogen on the surface, thermal diffusion, formation of molecular hydrogen, and hydrogen desorption from the surface-are considered as independent random Markovian-type processes and are simulated using stochastic algorithms. The suggested numerical model is computationally efficient allowing calculations to be held on model surfaces with up to 106 active centers, which corresponds to interstellar grains of up to 0.4 μm in diameter. Test calculations of the efficiency of molecular hydrogen formation were held for the two models of the surfaces of interplanetary grains, namely, for the homogeneous surfaces of olivine and amorphous carbon. It was confirmed that the effective formation of molecular hydrogen in diffuse molecular clouds is possible only within a narrow range of dust-particle temperatures. A comparison with the numerical results of other authors is presented.