Physical Conditions in Interstellar Hydroxyl and Formaldehyde Clouds

Abstract

The physical conditions in interstellar clouds which contain the molecules OH and H₂CO have been derived from an investigation of the H, OH and H₂CO absorption spectra of Cas A, Cyg A and Tau A. The derived parameters the visual obscuration and ultra-violet radiation field. These clouds with neutral hydrogen densities in the range 2 to 70 cm^–3 are evidently the higher density part of the distribution of interstellar clouds found in other neutral hydrogen investigations. It is shown that the lifetime of a molecule in the clouds is 10^–5 to 10^–4 of the lifetime of a typical cloud; this requires that the molecules should be continually formed. Most theories of OH molecule formation are inapplicable in the conditions found in these clouds. The physical parameters presented here should provide the basis for realistic theories of the formation of interstellar molecules.