A NONLOCAL CALCULATION OF CIRCUMSTELLAR OH MASER EMISSION

We present calculations for circumstellar OH masers which explicitly account for the nonlocal interaction throughout the masing region. Excitation temperatures and the observed emission are calculated for all four ground-state maser lines. All other transitions, including far-infrared and excited state microwave transitions, are treated using a modified Sobolev approximation for the lowest 24 hyperfine levels. Calculations are performed within the context of a simplified, self-consistent 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 work (Collison & Nedoluha 1994) and agree with the conclusions of Alcock & Ross (1986b) who showed that observed profiles cannot be produced by the ''standard model'' description of a smooth, spherically symmetric, steady, windlike mass loss. Alternative descriptions of the circumstellar outflow are discussed, and it is shown that the observed maser line profiles cannot be explained by minor changes to the standard model. Instead, a combination of significant clumping, sporadic mass loss, and/or deviations from spherical symmetry are required. The sensitive dependence of the observed emission on the physical conditions within the circumstellar shell implies that detailed information can be obtained by self-consistent modeling of both the outflow and the OH masers. Such models will require a method for solving the maser radiative transfer within a clumpy, possibly turbulent, medium.