PUMPING OF MAIN LINE OH MASERS IN THE CIRCUMSTELLAR ENVELOPES OF OH/INFRARED STARS

Models are presented for the pumping of the main line OH masers in the circumstellar shells of optically thick OH/IR stars. The effects of nonlocal far-IR (FIR) line overlaps, collisional excitations, and dust FIR pumping are included in the calculations of the level populations of OH. We also investigate the effect of a near-IR (NIR) overlap and show that an overlap with a strong transition of H2O is possible. These calculations are carried out within the context of a simple model of the outflow and thus incorporate the variation of pumping conditions with radius. The resulting population distribution gives the spatial location and relative strengths of the four ground state maser lines of OH. A comparison is made to recent observations of main line emission from OH/IR stars. We find general qualitative agreement between the models and these observations. Though for thick shells FIR overlaps alone appear to be able to reproduce the observations, a NIR overlap may be necessary to explain the observed emission from thinner shells, as has been suggested previously for type I Mira variables (Collision and Nedoluha 1993).