  Shock-induced H2O masers are important magnetic field tracers at very high
density gas. Water masers are found in both high- and low-mass star-forming
regions, acting as a powerful tool to compare magnetic field morphologies in
both mass regimes. In this paper, we show one of the first magnetic field
determinations in the low-mass protostellar core IRAS 16293-2422 at volume
densities as high as 10^(8-10) cm^-3. Our goal is to discern if the collapsing
regime of this source is controlled by magnetic fields or other factors like
turbulence. We used the Very Large Array (VLA) to carry out
spectro-polarimetric observations in the 22 GHz Zeeman emission of H2O masers.
From the Stokes V line profile, we can estimate the magnetic field strength in
the dense regions around the protostar. A blend of at least three maser
features can be inferred from our relatively high spatial resolution data set
(~ 0.1"), which is reproduced in a clear non-Gaussian line profile. The
emission is very stable in polarization fraction and position angle across the
channels. The maser spots are aligned with some components of the complex
outflow configuration of IRAS 16293-2422, and they are excited in zones of
compressed gas produced by shocks. The post-shock particle density is in the
range of 1-3 x 10^9 cm^-3, consistent with typical water masers pumping
densities. Zeeman emission is produced by a very strong line-of-sight magnetic
field (B ~ 113 mG). The magnetic field pressure derived from our data is
comparable to the ram pressure of the outflow dynamics. This indicates that the
magnetic field is energetically important in the dynamical evolution of IRAS
16293-2422.
