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V.L. Fish (Harvard-Smithsonian Center for Astrophysics)
We present full-polarization VLBA images of the 1665 and 1667~MHz OH maser transitions for several ultracompact (UC) \ion{H}{2} regions. Essentially all of the sources that have been mapped so far with the VLBA appear to have an organized magnetic field. About half exhibit one reversal in the line-of-sight direction of the magnetic field across the source. In most cases, the distribution of maser spots is too complicated to permit understanding in the context of a simple expansion model. However, collinear and filamentary structures are seen to exist, which may be tracers of shock fronts or disks. Maser linear polarization fractions and directions vary greatly both between sources and within the same source. When collinear maser spot formations are highly linearly polarized, the polarization direction is preferentially aligned with the collinear formation. These observations may provide additional constraints useful for theoretical interpretation of OH maser emission.
We also present observations of ground-state interstellar OH masers around UC\ion{H}{2} regions using the VLA. Magnetic fields deduced from Zeeman splitting have allowed us to investigate the large-scale magnetic field in the Galactic disk. Our data do not support a uniform circular Galactic-scale field. While we cannot conclusively identify magnetic field reversals between spiral arms, the magnetic fields around UC\ion{H}{2} regions appear to show some correlation on a scale of about 2 kiloparsecs.
Finally, we present the results of \ion{H}{1} absorption observations against UC\ion{H}{2} regions. In addition to resolving the near/far kinematic distance ambiguity for 20 UC\ion{H}{2} regions, we find that the HWHM of the height distribution of UC\ion{H}{2} regions above and below the Galactic plane is 35 ±2~pc within the solar circle. Based on this, we present a statistical analysis of the accuracy of resolving the kinematic distance ambiguity of UC\ion{H}{2} regions using their Galactic latitude.
The author(s) of this abstract have provided an email address for comments about the abstract: vfish@cfa.harvard.edu
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.