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We report full polarimetric VLBA observations of water masers towards theTurner-Welch Object in the W3(OH) high-mass star forming complex. This objectdrives 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 theinnermost scales of protostellar disk-jet systems. The linearly polarizedemission from water masers provides clues on the orientation of the localmagnetic field, while the measurement of the Zeeman splitting from circularpolarization provides its strength. The water masers trace a bipolar, biconicaloutflow at the center of the synchrotron jet. Although on scales of a fewthousand AU the magnetic field inferred from the masers is on averageorientated along the flow axis, on smaller scales (10s to 100s of AU), we haverevealed 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 fromhaving a dominant component parallel to the outflow velocity in the pre-shockgas, 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 oforder of a few hundred of mG in the post-shock gas where the water masers areexcited (at densities of $10^9$ cm$^{-3}$). The general implication is that inthe undisturbed (i.e. not-shocked) circumstellar gas, the flow velocities wouldfollow closely the magnetic field lines, while in the gas shocked by theprostostellar jet the magnetic field would be re-configured to be parallel tothe shock front.

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