  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.

Astrophysics of Galaxies;Solar and Stellar Astrophysics