Molecular and Solid Hydrogen in Dense Interstellar Clouds

Abstract

Recent observational data point to a significant underabundance of neutral atomic hydrogen in dark interstellar clouds-suggesting extensive formation of molecular or solid hydrogen. Conditions for the formation of molecular hydrogen (gaseous and solid) at grain surfaces are discussed. Grain temperatures less than ^6.5° K appear to be required for the formation of molecular H 2 and ^4° K for solid H 2. It is shown that grain temperatures as low as this may be attained in the interiors of dense clouds, due to the shielding effect by grains of the external interstellar radiation field. The photodissociating radiation is absorbed in a thin shell, and the time scale for recombination of atoms-> molecules is 3 X 10 4-3 X 10 6 years for densities of 10 4-10 2 cm-3. It is therefore expected that almost all dense clouds will be molecular regions. The process of recombination lowers the density and temperature and may lead to contraction of the clouds and the beginnings of star formation.