A NONLOCAL CALCULATION OF CIRCUMSTELLAR OH MASER EMISSION

We present calculations for circumstellar OH masers which explicitly a ccount for the nonlocal interaction throughout the masing region. Exci tation temperatures and the observed emission are calculated for all f our ground-state maser lines. All other transitions, including far-inf rared and excited state microwave transitions, are treated using a mod ified Sobolev approximation for the lowest 24 hyperfine levels. Calcul ations are performed within the context of a simplified, self-consiste nt dust/outflow model which provides the pumping conditions and their variation with radius. Total velocity relaxation is implicitly assumed . We, find general agreement with the qualitative results of earlier w ork (Collison & Nedoluha 1994) and agree with the conclusions of Alcoc k & Ross (1986b) who showed that observed profiles cannot be produced by the ''standard model'' description of a smooth, spherically symmetr ic, steady, windlike mass loss. Alternative descriptions of the circum stellar outflow are discussed, and it is shown that the observed maser line profiles cannot be explained by minor changes to the standard mo del. Instead, a combination of significant clumping, sporadic mass los s, and/or deviations from spherical symmetry are required. The sensiti ve dependence of the observed emission on the physical conditions with in the circumstellar shell implies that detailed information can be ob tained by self-consistent modeling of both the outflow and the OH mase rs. Such models will require a method for solving the maser radiative transfer within a clumpy, possibly turbulent, medium.