Dynamical evidence for a magnetocentrifugal wind from a 20 M_⊙ binary young stellar object

摘要

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 ～20 M_⊙. Here we map ground-state λ7 mm SiO emission with the Very Large Array and track proper motions over 9 yr. 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 ～18 km s~(-1). An overlying distribution of λ1.3 cm H_2O 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 ～120 AU downstream. The arcs of ground-state SiO emission may mark the transition point to a shocked super-Alfvénic outflow.