Interstellar OH Masers: The Environment Around UCH 2 Regions and the Galactic Magnetic Field
Abstract
We present full-polarization VLBA images of the 1665 and 1667 MHz OH maser transitions for several ultracompact (UC) 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 UCH 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 UCH 2 regions appear to show some correlation on a scale of about 2 kiloparsecs. Finally, we present the results of H 1 absorption observations against UCH 2 regions. In addition to resolving the near/far kinematic distance ambiguity for 20 UCH 2 regions, we find that the HWHM of the height distribution of UCH 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 UCH 2 regions using their Galactic latitude.
- Publication:
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American Astronomical Society Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AAS...201.3403F