Snapshot of a magnetohydrodynamic disk wind

Abstract The formation of astrophysical objects of different nature and size, from black holes to gaseous giant planets, involves a disk-jet system, where the disk drives the mass accretion onto a central compact object and the jet is a fast collimated ejection along the disk rotation axis. Magnetohydrodynamic disk winds can provide the link between mass accretion and ejection, which is essential to ensure that the excess angular momentum is removed from the system and accretion onto the central object can proceed. However, up to now, we have been lacking direct observational proof of disk winds. This work presents a direct view of the velocity field of a disk wind around a high-mass forming star. Achieving a very high spatial resolution of ~0.05~au, our water maser observations trace the velocities of individual streamlines emerging from the disk orbiting the forming star. We find that, at a low elevation above the disk midplane, the flow co-rotates with its launch point in the disk, in agreement with magneto-centrifugal acceleration where the gas is flung away along the magnetic field line anchored to the disk. Beyond the co-rotation point, the flow rises spiraling around the disk rotation axis along a helical magnetic field. Our results are, presently, the clearest evidence for a magnetohydrodynamic disk wind, and show that water masers and high-mass forming stars provide a suitable combination of tracer and environment to allow us studying the disk-wind physics.