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V. L. Fish (NRAO), M. J. Reid (Harvard--Smithsonian CfA)
We report analysis of the polarization properties of ground-state OH masers observed in a sample of 18 Galactic massive star-forming regions. A wide range of polarization properties are observed. At one extreme is W75N, in which masers with linear polarization fractions of unity are observed including clear \pi components, and linear polarization position angles of nearby maser spots appear to be correlated. At the other extreme is W51 e1/e2, in which no appreciable linear polarization is detected and some maser spots have total polarization fractions less than unity. A typical interstellar OH maser source has properties intermediate to these two extremes: some spots with appreciable linear polarization fractions, but no obvious \pi components and no clear organization of linear polarization position angles.
We interpret these results in the context of Faraday rotation at the maser site. Two relevant length scales exist: the gain length over which radiation in an unsaturated maser is increased by a factor of e, and the amplification length (many gain lengths) over which maser amplification occurs. The average interstellar OH maser source has large (> 1 rad) Faraday rotation over the amplification length but small Faraday rotation over a single gain length. Linear polarization can survive during amplification, but the linear polarization position angle is randomized. This represents a critical point from which small changes in the amount of Faraday rotation can change the observed properties. If the Faraday rotation per gain length is a factor of several smaller, large polarization fractions may still be observed, and the position angles may still be correlated with the local magnetic field direction (as in W75N). If the Faraday rotation is a factor of several larger, linear polarization will be destroyed on the scale of a gain length, and depolarization may occur (as in W51 e1/e2).
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.