Observed interstellar OH and H2O maser lines at 18 cm and1.35 cm exhibit unusual polarization properties. The OH emitters with thehighest brightness temperatures, usually associated with H II regions, almostalways show a high degree of circular polarization. The H2O maser line,on the other hand, is rarely polarized, and then only linearly polarized.
The preference for circular polarization in the brightest OHsources was attributed by Litvak to the mechanism of parametric down-conversion.In this process the higher-frequency components of a Zeemansplit maser line are down-converted to lower-frequency components and toan electron cyclotron wave. This mechanism is shown to be too weak to beof importance in astrophysical masers.
The polarization properties of the OH and H2O masers are related to the physical conditions in the maser clouds. It is found that the magneticfield, the plasma, and trapped infrared lines in maser sources play animportant role in determining the polarization on the emitted radiation.
