Methanol maser polarization in W3(OH)

Abstract We present the first 6.7-GHz methanol maser linear polarization map of the extended filamentary maser structure around the compact H ii region W3(OH). The methanol masers show linear polarization of up to ∼8 per cent and the polarization angles indicate a magnetic field direction along the north–south maser structure. The polarization angles are consistent with those measured for the OH masers, taking into account external Faraday rotation toward W3(OH), and confirm that the OH and methanol masers are found in similar physical conditions. Additionally we discuss the Zeeman splitting of the 6.7-GHz methanol transition and present an upper limit of ∼22 mG for the magnetic field strength in the maser region. The upper limit is fully consistent with the field strengths derived from OH maser Zeeman splitting.