OH maser disc and magnetic field structure in AFGL 2591

We report on Multi-Element Radio-Linked Interferometer Network (MERLIN) polarization measurements of OH 1665- and 1667-MHz masers in the star-forming region, AFGL 2591. The masers lie in an elliptical region of angular diameter 1.5 arcsec near the continuum source VLA 3, which has been proposed as the source powering the bipolar outflow in this region. The elliptical distribution of masers is orthogonal to the outflow, and is consistent with the OH masers tracing a molecular torus of radius ∼750 au, inclined to the line of sight by 55°. The ring has a velocity gradient that is consistent with rotation about a central mass of 31 M⊙. Zeeman splitting indicates a magnetic field ranging from −1.6 to +3.8 mG. The field reverses direction on opposite sides of the maser disc, indicating a toroidal component. Polarization of the OH masers in this source is relatively low, typically ∼50 per cent at 1665 MHz. The linear polarization vectors are predominantly either perpendicular or parallel to the outflow direction. The data are interpreted in terms of a twisted magnetic field model.