A MERLIN Study of 6-GHz excited-state OH and 6.7-GHz methanol masers in ON1

Multi-Element Radio Linked Interferometer Network (MERLIN) observations of 6.668-GHz methanol and both 6.031- and 6.035-GHz hydroxyl (OH) emission from the massive star formation region ON1 are presented. These are the first methanol observations made in full polarization using five antennas of MERLIN, giving high resolution and sensitivity to extended emission. Maser features are found to lie at the southern edge of the ultracompact H ii region, following the known distribution of ground-state OH masers. The masers cover a region ∼1 arcsec in extent, lying perpendicular to the H13CO+ bipolar outflow. Excited-state OH emission demonstrates consistent polarization angles across the strongest linearly polarized features which are parallel to the overall distribution. The linear polarizations vary between 10.0 and 18.5 per cent, with an average polarization angle of −60°± 28°. The strongest 6.668-GHz methanol features provide an upper limit to linear polarization of ∼1 per cent. Zeeman splitting of OH shows magnetic fields between −1.1 and −5.8 mG, and a tentative methanol magnetic field strength of −18 mG is measured.