Methanol and excited OH masers towards W51: I - Main and South
  MERLIN phase-referenced polarimetric observations towards the W51 complex
were carried out in March 2006 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 3 sigma limit of ~20 mJy per beam. Excited OH maser emission at 6.035
GHz is only found towards W51 Main. This emission is highly circularly
polarised (typically 45% and up to 87%). Seven Zeeman pairs were identified in
this transition, one of which contains detectable linear polarisation. 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"x2.2" (i.e.,
~16200x11900 AU), while only two maser components, separated by ~2.5", 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 2 regions showing a similar conical opening angle with of a
central velocity of ~+55.5 km/s and an expansion velocity of =<5 km/s. The mass
contained in this structure is estimated to be at least 22 solar masses. The
location of maser emission in the two afore-mentioned lines is compared with
that of previously published OH ground-state emission. Association with the
UCHII regions in the W51 Main/South complex and relationship of the masers to
infall or outflow in the region are discussed.
