Methanol and excited OH masers towards W51: I - Main and South

MERLIN phase-referenced polarimetric observations towards the W51 complexwere carried out in March 2006 in the Class II methanol maser transition at6.668 GHz and three of the four excited OH maser hyperfine transitions at 6GHz. Methanol maser emission is found towards both W51 Main and South. We didnot detect any emission in the excited OH maser lines at 6.030 and 6.049 GHzdown to a 3 sigma limit of \ {}20 mJy per beam. Excited OH maser emission at 6.035GHz is only found towards W51 Main. This emission is highly circularlypolarised (typically 45\% and up to 87\%). Seven Zeeman pairs were identified inthis transition, one of which contains detectable linear polarisation. Themagnetic field strength derived from these Zeeman pairs ranges from +1.6 to+6.8 mG, consistent with the previously published magnetic field strengthsinferred from the OH ground-state lines. The bulk of the methanol maseremission is associated with W51 Main, sampling a total area of \ {}3"x2.2" (i.e.,\ {}16200x11900 AU), while only two maser components, separated by \ {}2.5", arefound in the W51 South region. The astrometric distributions of both 6.668-GHzmethanol and 6.035-GHz excited-OH maser emission in the W51 Main/South regionare presented here. The methanol masers in W51 Main show a clear coherentvelocity and spatial structure with the bulk of the maser componentsdistributed into 2 regions showing a similar conical opening angle with of acentral velocity of \ {}+55.5 km/s and an expansion velocity of =\<5 km/s. The masscontained in this structure is estimated to be at least 22 solar masses. Thelocation of maser emission in the two afore-mentioned lines is compared withthat of previously published OH ground-state emission. Association with theUCHII regions in the W51 Main/South complex and relationship of the masers toinfall or outflow in the region are discussed.