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Methanol and excited OH masers towards W51 – I. Main and South




MERLIN phase-referenced polarimetric observations towards the W51 complex were carried out in 2006 March in the Class II methanol maser transition at 6.668 GHz and three of the four excited OH maser hyperfine transitions at 6 GHz. Methanol maser emission is found towards both W51 Main and South. We did not detect any emission in the excited OH maser lines at 6.030 and 6.049 GHz down to a 3σ limit of ∼20 mJy beam−1. Excited OH maser emission at 6.035 GHz is only found towards W51 Main. This emission is highly circularly polarized (typically ≥45 per cent and up to 87 per cent). Seven Zeeman pairs were identified in this transition, one of which contains detectable linear polarization. The magnetic field strength derived from these Zeeman pairs ranges from +1.6 to +6.8 mG, consistent with the previously published magnetic field strengths inferred from the OH ground-state lines. The bulk of the methanol maser emission is associated with W51 Main, sampling a total area of ∼3 × 2.2 arcsec2 (i.e. ∼16 200 × 11 900 au), while only two maser components, separated by ∼2.5 arcsec, are found in the W51 South region. The astrometric distributions of both 6.668-GHz methanol and 6.035-GHz excited-OH maser emission in the W51 Main/South region are presented here. The methanol masers in W51 Main show a clear coherent velocity and spatial structure with the bulk of the maser components distributed into two regions showing a similar conical opening angle with a central velocity of ∼+55.5 km s−1 and an expansion velocity of ≤5 km s−1. The mass contained in this structure is estimated to be at least 22 M. The location of maser emission in the two aforementioned lines is compared with that of previously published OH ground-state emission. The association with the ultra-compact H ii regions in the W51 Main/South complex and the relationship of the masers to infall or outflow in the region are discussed.