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 n onlocal 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 s how 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 w ith radius. The resulting population distribution gives the spatial lo cation 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 mo dels and these observations. Though for thick shells FIR overlaps alon e appear to be able to reproduce the observations, a NIR overlap may b e necessary to explain the observed emission from thinner shells, as h as been suggested previously for type I Mira variables (Collision and Nedoluha 1993).