A MODEL FOR OH MASERS IN STAR-FORMING REGIONS

We describe in detail a new model of the OH maser environment in star-forming regions and compare the predictions of the model with observational data. After reviewing the status of OH maser observations we outline the ingredients which such a model requires. Among these we highlight rate coefficients for energy transfer, the nature of the FIR dust radiation field and the treatment of line and continuum radiation transport in the presence of FIR line overlap where the latter is based on a new theoretical description. We derive and solve the steady-state rate equations using, the large velocity gradient approximation for 48 OH hyperfine level populations for a broad range of physical conditions. Concerning ourselves exclusively with unsaturated gain in OH, and thus with weak masers, we compare our predictions with VLA observations by Gaume & Mutel for ground-state (18 cm) masing action and we show that the effects of line overlap in accelerating outflows (or inflows) are crucial in order to reproduce the most important qualitative features of these data, e.g. the preponderance of 1665-MHz emission. We then consider excited-state masers with special reference to observed correlations between excited- and ground-state maser action. We are successful in reproducing these correlations, including for example that between 4765 MHz in F2(1/2) and 1720 MHz in the ground state. We are also able to reproduce the observed behaviour of the F1(7/2) excited state, predicting strong masing action only at 13 441 MHz. Our results show that the observational properties of OH masers are consistent with a moderately warm, dense environment sustaining bulk flows of material. These flows may represent the inner regions of outflows whose outer regions have been observed in a number of massive star-forming regions. We emphasize the qualitative nature of our results and suggest that further progress requires more detailed observational data on the spectral characteristics of the FIR field in star-forming regions, a more sophisticated means of treating the radiation transport with more realistic velocity fields, and more accurate rate coefficients for collisional energy transfer.