Effective grain surface area in the formation of molecular hydrogen in interstellar clouds

Abstract

Aims. In the interstellar clouds, molecular hydrogens are formed from atomic hydrogen on grain surfaces. An atomic hydrogen hops around till it finds another one with which it combines. This necessa rily implies that the average recombination time, or equivalently, the effective grain surface area depends on the relative numbers of atomic hydrogen influx rate and the number of sites on the grain. Our a im is to discover this dependency. Methods. We perform a numerical simulation to study the recombination of hydrogen on grain surfaces in a variety of cloud conditions. We use a square lattice (with a periodic boundary condition) of various sizes on two types of grains, namely, amorphous carbon and olivine. Results. We find that the steady state results of our simulation match v ery well with those obtained from a simpler analytical consideration ,