Full-Polarization Observations of OH Masers in Massive Star-Forming   Regions: II. Maser Properties and the Interpretation of Polarization
  We analyze full-polarization VLBA data of ground-state, main-line OH masers in 18 massive star-forming regions previously presented in a companion paper. The OH masers often arise in the shocked neutral gas surrounding ultracompact Hii regions. Magnetic fields as deduced from OH maser Zeeman splitting are highly ordered, both on the scale of a source as well as the maser clustering scale of ~10^15 cm. Results from our large sample show that this clustering scale appears to be universal to these masers. OH masers around ultracompact Hii regions live ~10^4 years and then turn off abruptly, rather than weakening gradually with time. These masers have a wide range of polarization properties. At one extreme (e.g., W75 N), pi-components are detected and the polarization position angles of maser spots show some organization. At the other extreme (e.g., W51 e1/e2), almost no linear polarization is detected and total polarization fractions can be substantially less than unity. A typical source has properties intermediate to these two extremes. In contrast to the well ordered magnetic field inferred from Zeeman splitting, there is generally no clear pattern in the distribution of polarization position angles. This can be explained if Faraday rotation in a typical OH maser source is large on a maser amplification length but small on a single (e-folding) gain length. Increasing or decreasing Faraday rotation by a factor of ~5 among different sources can explain the observed variation in polarization properties. We suggest that almost all pi-components acquire a signficant amount of circular polarization from low-gain stimulated emission of a sigma-component from OH appropriately shifted in velocity and lying along the propagation path. 
Vincent L. Fish and Mark J. Reid