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 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 rad m −2 , which would twist the J  = 2–1 position angles typically by less than 10°. 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 mG. 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 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.