Magnetic Field Structure of Orion Source I
Abstract
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 20 AU resolution, using the Very Large Array (VLA) and Atacama Large Millimeter/Submillimeter Array (ALMA), respectively. The 28SiO 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 104 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 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 28SiO 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 29SiO v=0 J=2-1 maser and several other SiO transitions at higher vibrational levels and isotopologues.