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R.L. Plambeck, M.C.H. Wright (U.C. Berkeley), R. Rao (U. Chicago)
To probe the magnetic field morphology near the massive young star Orion-IRc2, we used the BIMA array to map the linear polarization of the J=2-1 SiO masers, in both the v=1 and v=0 vibrational levels, with 0.4'' angular resolution.
The strong v=1 masers are confined to a narrow zone 40 AU from the star. These masers have fractional polarizations of a few percent. The polarization position angle tends to correlate with the maser intensity, and fluctuates on time scales of years. For the v=1 masers the stimulated emission rate R is much greater than the Zeeman splitting g\Omega due to any plausible magnetic field, so the maser polarization need not be correlated with the field direction.
The much weaker v=0 masers originate 500 AU from IRc2, in what appears to be a flared disk. For these masers we measure a position angle PA~80\degr, constant in time and varying little across the line profile. By contrast, Tsuboi et al. (1996) measured PA~150\degr\ for the J=1-0 v=0 SiO transition. Possibly this discrepancy is attributable to Faraday rotation by plasma in the foreground \ion{H}{2} region. If so, the intrinsic polarization angle is ~55\degr, parallel to the plane of the circumstellar disk. If the disk is seen edge-on, then the magnetic field threads the disk poloidally, with relatively little pinch.
This work was supported by NSF grant AST-21795.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.