OH masers and magnetic fields in the bipolar outflow source W75N

Abstract

MERLIN polarization measurements of 1665-, 1667- and 1720-MHz OH masers in the bipolar outflow source W75N are presented. The OH masers are distributed over a region 5 arcsec across (10 000 au at 2 kpc), which is elongated in a north-west-south-east (NW-SE) direction. The distribution is interpreted in terms of a rotating disc orthogonal to the bipolar outflow that is seen in CO and H2. A compact cluster of OH masers with a large velocity spread appears to mark the source of the outflow. Most of the OH masers detected have strong circular polarization up to 100 per cent. Seven Zeeman groups are identified, including the first example of a complete Zeeman triplet (two σ-components and one π-component). The field strength measured from Zeeman pairs of opposite circular polarization ranges from -8 to + 8 mG, with a field reversal on opposite sides of the disc. This is interpreted in terms of a toroidal component of the magnetic field in the disc. Linear polarization is also detected in many of the OH masers with some 1665-MHz masers showing up to 100 per cent linear polarization. The polarization position angles are preferentially along the direction of the bipolar outflow or perpendicular to it. The polarization data lend support to the twisted magnetic field model originally proposed by Uchida & Shibata.