FARADAY DEPOLARIZATION AND CIRCUMSTELLAR SIO MASERS

Calculations are performed for the creation and transport of linearly polarized maser radiation in the presence of Faraday depolarization du e to free electrons. Attention is limited to magnetic field strengths for which the characteristic Zeeman splitting g Omega is much less tha n the spectral line breadth Delta omega-the regime that ordinarily is appropriate for astrophysical SiO, H2O, and CH3OH masers. Analytic sol utions are obtained for unsaturated masers though numerical methods mu st be utilized when there is radiative saturation. The investigation h ere is motivated by observations of the differences in the fractional linear depolarization between the J = 3-2, 2-1, and 1-0 transitions of circumstellar SiO masers. These data have been interpreted as suggest ive of Faraday depolarization, with potential implications for the mag netic held strengths and fractional ionizations in these environments. For comparison with observation, the emphasis is on the relationship between the fractional polarization and the position angle of the pola rization vector. The changes in position angle that we calculate for F araday depolarization exclude this interpretation. Most likely, the ob served variations in the fractional polarization are due to a combinat ion of (1) differences in the degrees of saturation, (2) the detailed effect of anisotropic radiation on the multilevel network of the rate equations that govern the molecular populations, and (3) the competiti ve gain among the various masing transitions.