Volume 565, May 2014
|Number of page(s)||8|
|Published online||22 May 2014|
Rapidly increasing collimation and magnetic field changes of a protostellar H2O maser outflow⋆
1 Joint Institute for VLBI in Europe, Postbus 2, 7 9990 AA Dwingeloo, The Netherlands
2 Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
3 Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
4 Institut de Ciències de l’Espai (CSIC)-Institut de Ciències del Cosmos (UB)/IEEC, 08028 Barcelona, Spain
5 Instituto de Astronomía (UNAM), Apdo Postal 70-264, Cd. Universitaria, 04510-Mexico D.F., Mexico
6 Korea Astronomy and Space Science Institute, 776 Daedeokdaero, Yuseong, 305-348 Daejeon, Republic of Korea
7 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
Received: 25 March 2014
Accepted: 30 April 2014
Context. 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.
Aims. 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.
Methods. 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).
Results. 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.
Key words: stars: formation / masers / polarization / magnetic fields / ISM: individual objects: W75N
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2014
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