High-angular-resolution observations of the OH masers in W 51 (Main).

The authors have mapped the W51 ground-state OH masers using a VLBI array. The masers are clustered in two separate regions, one of which is coincident with an ultracompact H II region. Both maser clusters are nearly 3×10¹⁶cm in diameter. Two pairs of Zeeman split masers are identified in the 1720-MHz transition and indicate magnetic field strengths of 6 and 10 mG at two locations 500 AU apart in the compact H II region G49.5-0.4(e₂).