EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions^⋆

¹ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: gsurcis@astro.uni-bonn.de
² Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
⁵ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ National Astronomical Research Institute of Thailand, Ministry of Science and Technology, Rama VI Rd., 10400 Bangkok, Thailand

Received: 16 December 2011
Accepted: 14 March 2012

Abstract

Context. The role of magnetic fields in the formation of high-mass stars is still under debate, and recent measurements of their orientation and strength by using polarized maser emissions are contributing new insights. Masers polarization, in particular of the 6.7-GHz methanol masers, are one of the best probes of the magnetic field morphologies around massive protostars.

Aims. Determining the magnetic field morphology around an increasing number of massive protostars at milliarcsecond resolution by observing 6.7-GHz methanol masers is crucial to better understand the role of magnetic fields in massive star formation.

Methods. The First EVN Group consists of 4 massive star-forming complexes: W51, W48, IRAS 18556+0138, and W3(OH). These contain well-studied H ii regions from some of which molecular bipolar outflows were also detected (W51–e2, G35.20-0.74N). Nine of the European VLBI Network (EVN) antennas were used to measure the linear polarization and Zeeman-splitting of the 6.7-GHz methanol masers in the star-forming regions of the First EVN Group.

Results. We detected a total of 154 CH₃OH masers, one third of these towards W3(OH). Fractional linear polarization (1.2–11.5%) was detected towards 55 CH₃OH masers. The linear polarization vectors are well-ordered in all the massive star-forming regions. We measured significant Zeeman-splitting in 3 massive star-forming regions (W51, W48, and W3(OH)) revealing a range of separations  −3.5 m s^-1  <  ΔV_z < 3.8 m s^-1 with the smallest |ΔV_z| = 0.4 m s^-1. We were also able to compare our magnetic field results with those obtained from submillimeter wavelength dust observation in W51 and show that the magnetic field at low and high resolutions are in perfect agreement.