PUMPING OF MAIN LINE OH MASERS IN THE CIRCUMSTELLAR ENVELOPES OF TYPE-I MIRA VARIABLES

We have investigated the pumping of OH masers in the circumstellar envelopes of Mira variables. The location of the masing OH and the physical conditions within the circumstellar shell are approximated from a simple, spherically symmetric model of the mass loss. Parameters for the models are chosen to reproduce the conditions within Mira variables which show Type I (main line dominant) maser emission. The level populations of OH are calculated using the escape probability formalism and a modified Sobolev approximation. Pumping effects from direct stellar radiation, collisions, dust FIR emission, and FIR line overlaps are included. We find that throughout a seemingly reasonable range of conditions within the circumstellar shell, our models are dominated by 1612 MHz emission. Unless the conditions within the masing region are very different from those which we have assumed, we conclude that the FIR overlap mechanism alone cannot be responsible. for pumping the OH main lines. A possible pump mechanism which does produce Type I emission may be provided by the absorption of stellar NIR photons due to another molecule (possibly H2O) closer to the star.