Measuring Magnetic Fields from Water Masers in the Synchrotron Protostellar Jet in W3(H$_2$O)

TL;DR

This study uses polarimetric VLBA observations of water masers in the W3(H2O) high-mass star-forming region to investigate magnetic field structures and strengths in the jet and outflow regions, revealing magnetic field evolution due to shocks.

Contribution

It provides the first detailed measurement of magnetic field orientation and strength at small scales in a high-mass protostellar jet with associated water masers, highlighting magnetic field evolution from pre- to post-shock regions.

Findings

Magnetic fields are aligned with the outflow on large scales.

Field strength increases from tens to hundreds of mG across shock fronts.

Magnetic field reconfiguration occurs due to shock compression in the jet environment.

Abstract

We report full polarimetric VLBA observations of water masers towards the Turner-Welch Object in the W3(OH) high-mass star forming complex. This object drives a synchrotron jet, which is quite exceptional for a high-mass protostar, and is associated with a strongly polarized water maser source, W3(H$_2$O), making it an optimal target to investigate the role of magnetic fields on the innermost scales of protostellar disk-jet systems. The linearly polarized emission from water masers provides clues on the orientation of the local magnetic field, while the measurement of the Zeeman splitting from circular polarization provides its strength. The water masers trace a bipolar, biconical outflow at the center of the synchrotron jet. Although on scales of a few thousand AU the magnetic field inferred from the masers is on average orientated along the flow axis, on smaller scales (10s to 100s of AU), we have revealed a misalignment between the magnetic field and the velocity vectors, which arises from the compression of the field component along the shock front. Our measurements support a scenario where the magnetic field would evolve from having a dominant component parallel to the outflow velocity in the pre-shock gas, with field strengths of the order of a few tens of mG (at densities of $10^7$ cm$^{-3}$), to being mainly dominated by the perpendicular component of order of a few hundred of mG in the post-shock gas where the water masers are excited (at densities of $10^9$ cm$^{-3}$). The general implication is that in the undisturbed (i.e. not-shocked) circumstellar gas, the flow velocities would follow closely the magnetic field lines, while in the gas shocked by the prostostellar jet the magnetic field would be re-configured to be parallel to the shock front.