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VLBI observations of 6 GHz OH masers in three ultra-compact H II regions
J.F. Desmurs 1, 2, 3 and
A. Baudry 2
Received 30 July 1998 / Accepted 14 September 1998
Following our successful analysis of VLBI observations of the , , and excited OH emission at 6035 and 6031 MHz in W3(OH), we have analyzed the same transitions in three other ultra-compact HII regions, M17, ON1, and W51. The restoring beams were in the range 6 to 30 milliarc sec. The and 2-2 hyperfine transitions of OH were both mapped in ON1. Seven 6035 MHz LCP or RCP maser components were identified in ON1. They are distributed over a region whose diameter is similar to that of the compact HII region, namely arc sec. In contrast with the line emission, the transition at 6031 MHz is nearly an order of magnitude weaker than the peak 6035 MHz emission. In M17, we observed fringes only in the 6035 MHz line. The detected OH components appear to be projected on to the compact HII region. We report also on weak VLBI detection of the 6035 MHz emission from W51. This emission seems to be located between two active ultra-compact HII regions in a complex area which deserves further investigation. The 5 cm OH minimum brightness temperatures range from about 3 K in W51 to 8 K in ON1. Variability of the 6035 or 6031 MHz emission is well established and suggests that the 5 cm OH masers are not fully saturated.
The high spectral and spatial resolutions achieved in this work allowed us to identify Zeeman pairs and hence to derive the magnetic field strength. In ON1 and W51 the field lies in the range 4 to 6 mG with a trend for higher field at 6031 MHz than at 6035 MHz in ON1. In M17 no Zeeman splitting was observed and the magnetic field appears to be weaker than 1 mG.
Key words: masers techniques: interferometric ISM: magnetic fields ISM: individual objects: M17 ISM: individual objects: ON1 ISM: individual objects: W51
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Online publication: November 9, 1998