A milliarcsecond-resolution image of the OH masers in G34.3+0.2

We present measurements of the distribution of the OH masers at 1665 and 16 67 MHz towards the cometary ultracompact H II region in the complex G34.3+0 .2. The results are based on observations made in both senses of circular p olarization with a very long baseline interferometry (VLBI) array having an angular resolution of 5x20 mas(2). 38 maser features are identified in the region. 33 of these lie on an arc at the edge of the cometary H II region. Five are located in a cluster offset toward the north-east by 3 arcsec, an d are probably associated with an independent ultracompact H II region. The re is a velocity gradient of 30 km s(-1) pc(-1) across the arc. We identify five Zeeman pairs and determine that the magnetic field varies between 1 a nd 7 mG, but is always directed away from the Earth. The OH masers may arise in clumps in a shell of gas in a bow shock caused b y the motion of the exciting star through the molecular cloud. The stand-of f distance and the thickness of the shocked shell are roughly consistent wi th those predicted by such a bow-shock model. Also, the position of the exc iting star(s), as estimated from the focus of the parabolic bow shock, clos ely matches that of the peak emission from the cometary H II region. Howeve r, the north-south velocity gradient in the ionized material remains diffic ult to explain in the context of the bow-shock model.