Dynamical Evidence for a Magnetocentrifugal Wind from a 20 Msun Binary Young Stellar Object

TL;DR

This study provides dynamical evidence for a magnetocentrifugal disk wind from a 20 solar mass binary young stellar object, using long-term proper motion mapping of SiO and H2O masers.

Contribution

It presents the first detailed proper motion analysis over nine years that supports the presence of a magnetocentrifugal disk wind in a massive YSO.

Findings

Proper motions reveal a bipolar outflow with median 3D speed of ~18 km/s.

Flow morphology and maser data suggest a magnetocentrifugal disk-wind mechanism.

Evidence of flow colinearity, rotation, and recollimation supports the wind model.

Abstract

In Orion BN/KL, proper motions of 7 mm vibrationally-excited SiO masers trace 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 (YSO) of ~20 Msun. Here we map ground-state 7 mm SiO emission with the Very Large Array and track proper motions over 9 years. 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 3D motion of ~18 km/s. An overlying distribution of 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 ~120 AU downstream. The arcs of ground-state SiO emission may mark the transition point to a shocked super-Alfvenic outflow.