Evolution of 1612-MHz maser emission in expanding circumstellar shells

Observations show that 1612-MHz masers of OH/IR stars can fade on a time-scale of a decade. This fading is probably associated with the switch from rapid mass loss, which is ultimately linked with an internal He-shell flash, to the much slower mass loss supported by more quiescent conditions. We study the observed maser decay with a composite computational model, comprising a time-dependent chemical model of the envelope, and a radiation transfer model which provides the maser pumping. Our combined model is able to reproduce the rapid decay of maser intensity, following a sudden drop in the stellar mass-loss rate. The explanation for the rapid fall in maser emission is not a fall in the OH number density, or the kinetic temperature in the inverted layers, but the loss of a radiative pump route which carries population from levels 1 to 4 via levels 16, 15 and 11. The loss of these pump routes is a result, in turn, of a greatly reduced energy density of 53-μm radiation.