Elementary Models for Stimulated Radio Emission from Interstellar OH

Abstract

The aim of this paper is to explore, on a phenomenological basis, some implications of the hypothesis that the 18-cm radiation from interstellar OH arises from stimulated emission. In order to have a reasonably well-defined model, it is supposed that the radiating gas lies in the ionization front around an H II region and that stimulated emission occurs only from narrow columns with well-defined gas velocities. The total radio power from an H II region is estimated on a statistical basis and compared with the power radiated by an O or B star at the centre of the region. Estimates are also made of the characteristic length of a radiating column, of the gain of the equivalent stimulated emission amplifier and of the necessary population inversion. To see if the observed peculiar intensity ratios and time variations can be accounted for by stimulated emission, the coupled non-linear equations for radiation transfer and for the population ratios are formulated and studied in a preliminary way. It is shown that for a wide range of assumptions about the molecular processes involved in inverting the population, the observations cannot be accounted for if the stimulated terms can be ignored in comparison with the spontaneous terms in the rate equations. If the stimulated terms are retained (as seems in fact justified by the intensity of the radiation) the coupling between the two sets of equations may lead to intensity ratios such as are observed. The implications of these results for the molecular physics and for the structure of H II regions are indicated.