Rapidly increasing collimation and magnetic field changes of a H2O maser
  outflow
  W75N(B) is a massive star-forming region that contains three radio continuum
sources (VLA 1, VLA 2, and VLA 3), which are thought to be three massive young
stellar objects at three different evolutionary stages. VLA 1 is the most
evolved and VLA 2 the least evolved source. The 22 GHz H2O masers associated
with VLA 1 and VLA 2 have been mapped at several epochs over eight years. While
the H2O masers in VLA 1 show a persistent linear distribution along a radio
jet, those in VLA 2 are distributed around an expanding shell. Furthermore, H2O
maser polarimetric measurements revealed magnetic fields aligned with the two
structures. Using new polarimetric observations of H2O masers, we aim to
confirm the elliptical expansion of the shell-like structure around VLA 2 and,
at the same time, to determine if the magnetic fields around the two sources
have changed. The NRAO Very Long Baseline Array was used to measure the linear
polarization and the Zeeman-splitting of the 22 GHz H2O masers towards the
massive star-forming region W75N(B). The H2O maser distribution around VLA 1 is
unchanged from that previously observed. We made an elliptical fit of the H2O
masers around VLA 2. We find that the shell-like structure is still expanding
along the direction parallel to the thermal radio jet of VLA 1. While the
magnetic field around VLA 1 has not changed in the past 7 years, the magnetic
field around VLA 2 has changed its orientation according to the new direction
of the major-axis of the shell-like structure and it is now aligned with the
magnetic field in VLA 1.
