GEOMETRICAL EFFECTS IN MODELS OF OH IR-STAR MASERS/

We use a model presented earlier to address several outstanding questi ons about the 1612-MHz masers in OH/IR stars. First, we explore the pa rameter values for modelling the masers, and show that the appearance of the masers has no strong dependence on the shell thickness, OH dens ity or velocity distribution of the OH molecules. Secondly, we examine the possible mismatch between angular size and phase-lag size, which may affect distance estimates. Such an effect may arise because of the anisotropy of the maser emission, but is found to be small. Finally, we discuss the presence of maser emission of high brightness temperatu re in the blueshifted peak, and show that it is indeed possible that t his effect is caused by amplified stellar emission.