2006ApJS..164...99F

We analyze full-polarization VLBA data of ground-state OH masers in 18 massive star-forming regions previously presented in a companion paper. We confirm results previously seen in the few individual sources studied at milliarcsecond angular resolution. The OH masers often arise in the shocked neutral gas surrounding ultracompact H II regions. Magnetic fields as deduced from OH maser Zeeman splitting are highly ordered on the scale of a source and on the maser clustering scale of ∼10¹⁵ cm, which appears to be universal. OH masers around ultracompact H II regions live ∼10⁴ yr before turning off abruptly, rather than weakening gradually. These masers have a wide range of polarization properties. At one extreme (e.g., W75 N), π-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 partial depolarization occurs. A typical source has properties intermediate to these two extremes, with 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. Pure π-components (in theory, 100% linearly polarized) are seldom seen. We suggest that almost all π-components acquire a significant amount of circular polarization from low-gain stimulated emission of a σ-component from velocity-coherent OH lying along the propagation path.