MONTE-CARLO SOLUTIONS FOR THE RADIATIVE PROCESSES IN 1612-MHZ OH MASERS

We use a Monte Carlo method to solve the radiative transport in circumstellar OH masers. In this way we model the observed double-peaked 1612-MHz maser emission from expanding molecular shells around highly evolved stars. Earlier attempts to describe in detail the shape of the spectra have had limited success, because of the complexity of the radiative transfer problem, in that the radiation of the maser couples to the population inversion which - together with the global geometry of the model - determines the radiation field. It turns out that our approach deals with this automatically, with the big advantage that the whole problem is coded in terms of local physics. We show that our code can reproduce features like maser saturation and beam crossing, and we discuss its limits. With plausible parameter values from the literature we readily obtain spectra that resemble the observed spectra quite well. Thus we conclude that the average shape of OH/IR star spectra can be fully explained by a model of a spherically symmetric, homogeneous and constantly expanding shell.