Astronomy and Astrophysics, volume 340, 521-526 (1998/12-2)
VLBI observations of 6 GHz OH masers in three ultra-compact HII regions.
DESMURS J.F. and BAUDRY A.
Abstract (from CDS):
Following our successful analysis of VLBI observations of the 2 Π3/2, J5/2, F=3-3 and F=2-2 excited OH emission at 6035 and 6031MHz 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 milliarcsec. The F=3-3 and 2-2 hyperfine transitions of OH were both mapped in ON1. Seven 6035MHz 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 ≃0.4-0.5arcsec. In contrast with the F=3-3 line emission, the F=2-2 transition at 6031MHz is nearly an order of magnitude weaker than the peak 6035MHz emission. In M17, we observed fringes only in the 6035MHz line. The detected OH components appear to be projected on to the compact HII region. We report also on weak VLBI detection of the 6035MHz 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 3x107K in W51 to 8x109K in ON1. Variability of the 6035 or 6031MHz 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 6mG with a trend for higher field at 6031MHz than at 6035MHz in ON1. In M17 no Zeeman splitting was observed and the magnetic field appears to be weaker than 1mG.
masers - techniques: interferometric - ISM: magnetic fields - ISM: individual objects: M 17 - ISM: individual objects: ON 1 - ISM: individual objects: W51
Table 2: [DB98] M17 A (Nos A-B), [DB98] ON1 A (Nos A-D), [DB98] ON1 A' (No. A'), [DB98] W51 A (No. A).
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