Snapshot of a magnetohydrodynamic disk wind traced by water maser observations

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 a 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 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 Article number, page 1 of 26 L. Moscadelli * et al.: Snapshot of a magnetohydrodynamic disk wind

Modeling disks and magnetic outflows around a forming massive star

“…The most direct observational evidence of the magnetocentrifugal mechanism and the acceleration of the jet as described here is provided by the recent observations of the most massive young stellar object (M = 5.6 ± 2.0M ) in the starforming region IRAS 21078+5211 (Moscadelli et al 2022(Moscadelli et al , 2021. In Moscadelli et al (2022), 22 GHz water maser observations (which trace internal shocks of the gas), were able to uncover the helical trajectories of the plasma as it is magnetocentrifugally ejected.…”

“…The most direct observational evidence of the magnetocentrifugal mechanism and the acceleration of the jet as described here is provided by the recent observations of the most massive young stellar object (M = 5.6 ± 2.0M ) in the starforming region IRAS 21078+5211 (Moscadelli et al 2022(Moscadelli et al , 2021. In Moscadelli et al (2022), 22 GHz water maser observations (which trace internal shocks of the gas), were able to uncover the helical trajectories of the plasma as it is magnetocentrifugally ejected. In that article, we used the simulation corresponding to the fiducial case with grid x16 to produce a snapshot comparison when the protostellar mass is similar to the observed value, and found that the geometry and kinematics of the transition region where the flow abandons the Alfvén surface and becomes helical coincide very well with the observed masers.…”

“…5). Additionally, episodic accretion might cause small precession in the jet axis, as suggested by the different regions of the red-and blueshift in the ejected material at large and small scales in IRAS 21078+5211 (Moscadelli et al 2021(Moscadelli et al , 2022.…”

“…A new generation of observations that combine data from new or upgraded (sub)millimiter interferometers such as the Northern Extended Millimeter Array (NOEMA), the Atacama Large Millimeter/submillimeter Array (ALMA), the Very Long Baseline Array (VLBI) and the Jansky Very Large Array (JVLA) have enabled an unprecedented resolution of the innermost ∼ 100 au around the forming massive star such that the direct study of the dynamical processes that intervene in the launching (Moscadelli et al 2022) and collimation (Carrasco-González et al 2021) of the jet is now possible. However, previous numerical studies of massive protostellar outflows (for example, Rosen & Krumholz 2020;Machida & Hosokawa 2020or Mignon-Risse et al 2021; a more detailed literature review is offered in Sect.…”

“…Thanks to observations of water masers done in Moscadelli et al (2022), the innermost ∼ 100 au region where the jet is launched can now be observed with a resolution of up to 0.05 au. In that article, we presented a simulation of a forming massive star surrounded by a disk-jet system that was able to reach those resolutions and reproduced reasonably well the observed jet kinematics.…”