Magnetic Field Structure of Orion Source I
  We observed polarization of the SiO rotational transitions from Orion Source
I (SrcI) to probe the magnetic field in bipolar outflows from this high mass
protostar. Both 43 GHz $J$=1-0 and 86 GHz $J$=2-1 lines were mapped with
$\sim$20 AU resolution, using the Very Large Array (VLA) and Atacama Large
Millimeter/Submillimeter Array (ALMA), respectively. The $^{28}$SiO transitions
in the ground vibrational state are a mixture of thermal and maser emission.
Comparison of the polarization position angles in the $J$=1-0 and $J$=2-1
transitions allows us to set an upper limit on possible Faraday rotation of
$10^{4}$ radians m$^{-2}$, which would twist the $J$=2-1 position angles
typically by less than 10 degrees. The smooth, systematic polarization
structure in the outflow lobes suggests a well ordered magnetic field on scales
of a few hundred AU. The uniformity of the polarization suggests a field
strength of $\sim$30 milli-Gauss. It is strong enough to shape the bipolar
outflow and possibly lead to sub-Keplerian rotation of gas at the base of the
outflow. The strikingly high fractional linear polarizations of 80-90% in the
$^{28}$SiO $v$=0 masers require anisotropic pumping. We measured circular
polarizations of 60% toward the strongest maser feature in the $v$=0 $J$=1-0
peak. Anisotropic resonant scattering (ARS) is likely to be responsible for
this circular polarization. We also present maps of the $^{29}$SiO $v$=0
$J$=2-1 maser and several other SiO transitions at higher vibrational levels
and isotopologues.
