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Astron. Astrophys. 334, 1085-1094 (1998)

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VLBI maps and properties of the 6 GHz OH masers in W 3(OH)

J.F. Desmurs 1, A. Baudry 1, T.L. Wilson 2, R.J. Cohen 3 and G. Tofani 4

1 Observatoire de l'Université de Bordeaux 1, BP 89, F-33270 Floirac, France
2 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
3 Nuffield Radio Astronomy Laboratories, Jodrell Bank, Macclesfield, Cheshire, SK11 9DL, UK
4 Arcetri Observatory, Largo E.Fermi 5, I-50125 Firenze, Italy

Received 23 December 1997 / Accepted 18 March 1998


We present the first maps of the [FORMULA] and [FORMULA] OH maser emission from W3(OH) at 6.035 and 6.031 GHz in both right and left circular polarizations. We used three antennas of the European VLBI Network to achieve a spatial resolution of a few milliarcseconds (mas). Our maps, restored with a beam of 5 x 6.5 mas, show complex OH emission structures in several velocity channels. Weak extended emission structures could be present together with point-like sources. The minimum brightness temperature derived for individual maser spots lies in the range 0.2-5 [FORMULA] K. The 6.035 GHz maser emission is concentrated in five distinct regions covering the western half of the compact HII region. There are significantly fewer features at 6.031 GHz although the overall spatial distributions of OH features of the [FORMULA] and [FORMULA] transitions are similar.

Nearly all OH features with nearby center velocities and opposite senses of circular polarization coincide to within one synthesized beamwidth. We identify these spatially paired components with Zeeman pairs and derive the associated magnetic field strengths, for which some changes have been observed since the first experiment made by Moran et al. (1978). The field always points away from us. The strengths deduced from the 6.035 GHz data range from 2 to 10 mG. At 6.031 GHz the field strengths are also [FORMULA] mG with the exception of a strong feature around -42.6 km s-1 which gives [FORMULA] mG. This is the highest field strength measured so far in an OH line.

From our observations we derived the absolute position of the maser emission to an accuracy of order 200 mas for both 6.035 and 6.031 GHz transitions. The fine scale alignment of the [FORMULA] and [FORMULA] OH emission maps was made using kinematical and other physical arguments. In the richest area of individual OH spots there is a good match between the strongest 6.035 and 6.031 GHz masers which thus must be excited by similar physical processes. However, the conditions required to excite the 6.031 GHz maser seem to be slightly different from those at 6.035 GHz because the linewidths are narrower and the magnetic fields are stronger at 6.031 GHz.

Key words: masers – polarization – ISM: W 3(OH) – radio lines: ISM

Present address: JIVE / Observatorio Astronómico Nacional, Apartado 1143, E-28800 Alcalá de Henares, Spain

Send offprint requests to: desmurs@observ.u-bordeaux.fr

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998