OH maser polarization
The mechanism to produce circular polarization has been reinvestigated under conditions with radiative pumping and the Faraday rotation due to electrons. A new radiative pump mechanism is proposed for the OH main line masers. A parity propensity for the OH collisional excitation, which has recently been found by laboratory experiments, prohibits the collisional pumping of the main line masers. It is shown that the inversion can be created when the radiation temperature is higher than the kinetic temperature. The anisotropic radiative pump can produce unequal magnetic sublevel populations. In addition to the line overlap between the Zeeman triplet of the OH main lines, a modest amount of the Faraday rotation due to electrons can increase the degree of circular polarization of OH masers even when the angle to the magnetic filed is near π/2.
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