Total Linear Polarization in the OH Maser W75 N: VLBA Polarization Structure

Abstract

W75 N is a star-forming region containing various ultracompact H II regions and OH, H2O, and CH3OH maser emission. Our VLBA map shows that the OH masers are located in a thin disk rotating around an O star, the exciting star of the ultracompact H II region VLA 1. A separate set of maser spots is associated with the ultracompact H II region VLA 2. The radial velocity of OH maser spots varies across the disk from 3.7 to 10.9 km s-1. The diameter of the disk is 4000 AU. All maser spots are strongly polarized. These are the first OH masers showing nearly 100% linear polarization in several spots. Two maser spots seem to be Zeeman pairs, corresponding to a magnetic field of 5.2 and 7.7 mG, and in one case we tentatively find a Zeeman pair consisting of two linearly polarized components. The linearly polarized maser spots are shown to be σ components, which is the case when the magnetic field is perpendicular to the line of sight. The direction of the magnetic field as determined from linearly polarized spots is perpendicular to the plane of the disk, although the Galactic Faraday rotation may significantly affect this conclusion.