Methanol masers probing the ordered magnetic field of W75N^*

¹ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: gsurcis@astro.uni-bonn.de
² Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ International Centre for Radio Astronomy Research, University of Western Australia, Perth, Australia
⁴ Joint Institute for VLBI in Europe, Postbus 2, 79990 AA Dwingeloo, The Netherlands
⁵ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands

Received: 30 June 2009
Accepted: 17 July 2009

Abstract

Context. The role of magnetic fields during the protostellar phase of high-mass star-formation is a debated topic. In particular, it is still unclear how magnetic fields influence the formation and dynamic of disks and outflows. Most current information on magnetic fields close to high-mass protostars comes from H₂O and OH maser observations. Recently, the first 6.7 GHz methanol maser polarization observations were made, and they reveal strong and ordered magnetic fields.

Aims. The morphology of the magnetic field during high-mass star-formation needs to be investigated on small scales, which can only be done using very long baseline interferometry observations. The massive star-forming region W75N contains three radio sources and associated masers, while a large-scale molecular bipolar outflow is also present. Polarization observations of the 6.7 GHz methanol masers at high angular resolution probe the strength and structure of the magnetic field and determine its relation to the outflow.

Methods. Eight of the European VLBI network antennas were used to measure the linear polarization and Zeeman-splitting of the 6.7 GHz methanol masers in the star-forming region W75N.

Results. We detected 10 methanol maser features, 4 of which were undetected in previous work. All arise near the source VLA 1 of W75N. The linear polarization of the masers reveals a tightly ordered magnetic field over more than 2000 AU around VLA 1 that is exactly aligned with the large-scale molecular outflow. This is consistent with the twisted magnetic field model proposed for explaining dust polarization observations. The Zeeman-splitting measured on 3 of the maser features indicates a dynamically important magnetic field in the maser region of the order of 50 mG. We suggest VLA 1 is the powering sources of the bipolar outflow.