Saturation and spectral asymmetry in 1612 MHz shell masers

At least one 'dying' asymptotic giant branch star has been observed to show a strong spectral asymmetry in the decay of its 1612 MHz OH maser spectrum, with the red peak decaying substantially more rapidly than the blue peak. We have tested a model which explains the general behaviour of such stars to see if it can also explain asymmetry of this type. We find that it is actually very difficult to overcome a natural propensity for the blue peak to decay more rapidly, which is tied to the distribution of OH molecules and pumping radiation in the model. A possible solution is the presence of strongly saturating rays in the blue peak from a large stellar radiosphere, which decay more slowly than an almost unsaturated red peak. © 2008 RAS.