Normal OH Emission and Interstellar Dust Clouds

Normal emission from OH in interstellar dust clouds has been detected. The clouds have normal cosmic abundances; the hydrogen, which is not observed to emit 21-cm line radiation, is probably all H2. The amounts of OH and H2 are consistent with statistical equilibrium with known reactions. The clouds are sufficiently opaque to exclude nearly all of the galactic radiation field so that photodissociation does not occur; also, the resulting radiation pressure differential helps hold the clouds together