Physics of Molecular Clouds from Millimeter Wave Line Observations

Abstract

The advent of millimeter wave molecular line astronomy has made it possible over the past seven years for astronomers to study the physics and chemistry of dense interstellar clouds. The dominant gaseous constituent of these clouds is molecular hydrogen, H2, and hence the name molecular clouds. In fact all interstellar clouds with hydrogen density greater than 100 cm-3 are composed primarily of molecular and not atomic hydrogen. The dominance of molecules in dense clouds is due to the self shielding of H2 from the photodissociating starlight at the wavelengths of the ultraviolet Lyman bands. At a density of 100 cm-3 the production rate of H2 on grain surfaces is sufficient to build up a layer which is optically thick to the photodissociation occurring in the lines of the Lyman band. Inside the clouds, the photodissociation, which is limited to the line wings, is reduced by several orders of magnitude and the production rate dominates with almost all hydrogen converted to H2. Since H2 does not have a true continuum photodissociation at λ > 912 A, the self shielding of the bound-bound-free process of the Lyman bands is critical to the distinction between atomic and molecular clouds.