The properties and polarization of the H₂O  and CH₃OH  maser environment of NGC 7538-IRS 1

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, 7 9990 AA Dwingeloo, The Netherlands
³ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
⁴ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ National Astronomical Research Institute of Thailand, Ministry of Science and Technology, Rama VI Rd., 10400 Bangkok, Thailand

Received: 19 April 2011
Accepted: 20 July 2011

Abstract

Context. NGC 7538 is a complex massive star-forming region. The region is composed of several radio continuum sources, one of which is IRS 1, a high-mass protostar, from which a 0.3 pc molecular bipolar outflow was detected. Several maser species have been detected around IRS 1. The CH₃OH masers have been suggested to trace a Keplerian-disk, while the H₂O masers are almost aligned to the outflow. More recent results suggested that the region hosts a torus and potentially a disk, but with a different inclination than the Keplerian-disk that is supposed to be traced by the CH₃OH masers.

Aims. Tracing the magnetic field close to protostars is fundamental for determining the orientation of the disk/torus. Recent studies showed that during the protostellar phase of high-mass star formation the magnetic field is oriented along the outflows and around or on the surfaces of the disk/torus. The observations of polarized maser emissions at milliarcsecond resolution can make a crucial contribution to understanding the orientation of the magnetic field and, consequently, the orientation of the disk/torus in NGC 7538-IRS 1.

Methods. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman-splitting of the 22 GHz H₂O masers toward NGC 7538-IRS 1. The European VLBI Network and the MERLIN telescopes were used to measure the linear polarization and the Zeeman-splitting of the 6.7 GHz CH₃OH masers toward the same region.

Results. We detected 17 H₂O masers and 49 CH₃OH masers at high angular resolution. We detected linear polarization emission toward two H₂O masers and toward twenty CH₃OH masers. The CH₃OH masers, most of which only show a core structure, seem to trace rotating and potentially infalling gas in the inner part of a torus. Significant Zeeman-splitting was measured in three CH₃OH masers. No significant (3σ) magnetic field strength was measured using the H₂O masers. We also propose a new description of the structure of the NGC 7538-IRS 1 maser region.