Physical Adsorption Processes on Interstellar Graphite Grains

The conditions are examined under which mantles will form on interstellar graphite grains. They may form by the adsorption of molecules from the interstellar gas, and relations are given connecting the gas densities and grain temperatures for which mantle growth occurs. Graphite grains in quiescent H i regions will not grow mantles unless the grain temperature is less than about 20° K. In the atmos- pheres of cool stars where the molecule density is expected to be high, grains blown out of the atmosphere by radiation pressure have even higher temperatures and mantle growth is prevented until the inter- stellar case prevails Physically adsorbed atoms form quasi-molecules on the grains' surfaces which can interact with the lattice to form stable molecules if these resultant molecules have dipole moments on the order of a debye. I. INTRODUCTIO