2013ApJ...770L..32G

In Orion BN/KL, proper motions of λ7 mm vibrationally excited SiO masers trace the rotation of a nearly edge-on disk and a bipolar wide-angle outflow 10-100 AU from radio source I, a binary young stellar object of ∼20M_☉. Here we map ground-state λ7mm SiO emission with the Very Large Array and track proper motions over 9yr. The innermost and strongest emission lies in two extended arcs bracketing Source I. The proper motions trace a northeast-southwest bipolar outflow 100-1000 AU from Source I with a median three-dimensional motion of ∼18km/s. An overlying distribution of λ1.3cm H₂O masers betrays similar flow characteristics. Gas dynamics and emission morphology traced by the masers suggest the presence of a magnetocentrifugal disk wind. Reinforcing evidence lies in the colinearity of the flow, apparent rotation across the flow parallel to the disk rotation, and recollimation that narrows the flow opening angle ∼120AU downstream. The arcs of ground-state SiO emission may mark the transition point to a shocked super-Alfvénic outflow.