DYNAMICAL EVIDENCE FOR A MAGNETOCENTRIFUGAL WIND FROM A 20 M⊙ BINARY YOUNG STELLAR OBJECT

In Orion BN/KL, proper motions of lambda 7 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 similar to 20 M-circle dot. Here we map ground-state lambda 7 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 similar to 18 km s(-1). An overlying distribution of lambda 1.3 cm 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 similar to 120 AU downstream. The arcs of ground-state SiO emission may mark the transition point to a shocked super-Alfvenic outflow.