Rapidly increasing collimation and magnetic field changes of a H2O maser outflow

TL;DR

This study uses polarimetric observations of H2O masers in W75N(B) to confirm the elliptical expansion of a shell around VLA 2 and to analyze magnetic field changes over eight years, revealing stable fields around VLA 1 and a reoriented field around VLA 2.

Contribution

It provides new polarimetric measurements confirming shell expansion and magnetic field reorientation around VLA 2 in a massive star-forming region.

Findings

Shell around VLA 2 is still expanding.

Magnetic field around VLA 1 remains unchanged.

Magnetic field around VLA 2 has reoriented to align with VLA 1.

Abstract

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.