The Formation of Interstellar Molecular Hydrogen by Physical Adsorption on Grains

Abstract

The formation of molecular hydrogen on interstellar grains by means of physical adsorption will proceed efficiently if the adsorptive potential is high enough and the grain temperature is low enough. The physical-adsorption process is examined. It is shown that electrostatic forces produce high adsorptive potentials in the case of silicate and graphite materials and that dispersive forces dominate on ices. The effect of heterogeneity of adsorption sites is examined. Available laboratory data are analyzed to determine the adsorptive potential of H atoms on silicates, graphite, and solid H2 and 1120. If the structure and composition of grains in space are similar to the materials considered in this study and if current estimates of grain temperatures are correct, then interstellar molecular hydrogen should be formed by physical adsorption on grains. The formatina of H2 in the Ophiuchus and Taurus clouds is examined to see if different adsorptive potentials exist on the grains in these two clouds. The conclusion is reached, based on the adsorptive potential for H on solid H2 presented in this study, that if the formation of molecular hydrogen is occurring in these clouds it is apparently proceeding at the same rate in each cloud and that hydrogen mantles have not formed on the grains in either cloud.