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

Abstract

We present measurements of the distribution of the OH masers at 1665 and 1667 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 polarization with a very long baseline interferometry (VLBI) array having an angular resolution of 5 × 20 mas2. 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, and are probably associated with an independent ultracompact H II region. There 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 and 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 by the motion of the exciting star through the molecular cloud. The stand-off distance and the thickness of the shocked shell are roughly consistent with those predicted by such a bow-shock model. Also, the position of the exciting star(s), as estimated from the focus of the parabolic bow shock, closely matches that of the peak emission from the cometary H II region. However, the north-south velocity gradient in the ionized material remains difficult to explain in the context of the bow-shock model.