In the very late stages of AGB evolution, a star develops a super-wind with extreme mass loss. The gas in the stellar wind condenses as it cools away from the star, forming molecules such as water (H2O) and silicon monoxide (SiO). This can form grains of dust, mostly silicates, which obscure the star at shorter wavelengths, leading to a strong infrared source.[1]Hydroxyl (OH) radicals can be produced by photodissociation or collisional dissociation.[2]
H2O and OH can both be pumped to produce maser emission. OH masers in particular can give rise to a powerful maser action at 1612 MHz and this is regarded as a defining feature of the OH/IR stars. Many other AGB stars such as Mira variables show weaker OH masers at other wavelengths, such as 1667MHz or 22MHz.[3]
