1998A&A...331..697B

We have obtained the first polarization images of the J=7/2, F=4-4 OH maser emission from W3(OH) at 13.44GHz with high dynamic range and resolution. The emission has been imaged with a spectral resolution of 0.087km/s and with a spatial resolution of 670µarcsec, the highest ever achieved in any image of the OH radical. The 13.44GHz OH maser emission lies within an extremely compact area of 75x105milliarcsec (mas) centered on the edge of the northern ionized clump observed in high frequency continuum images of W3(OH). We determined the absolute position of the -42.89km/s reference feature to an accuracy of 0.01-0.02'' thus allowing us to register OH images taken at different epochs. The area of OH emission contains spatially and kinematically distinct subregions. Among them, we discovered a remarkable filament extending over ≃15mas (or 30AU). This filament is part of a larger scale arc structure nearly 60AU in size, the first arc-like structure ever observed in OH masers. A high velocity gradient and a magnetic field gradient are observed along this arc. Most of the OH flux density is contained at a position lying nearly at the center of the arc structure, and perhaps marking the site of the obscured O-type star that excites W3(OH). Our data show that elongated OH-emitting structures, perhaps related to local shocks, are dominant, and that no simple geometry exists for the individual OH maser spots. However, the observations made with the longest baselines are consistent with a size of about 0.4mas (or ≃0.9AU). This is the most compact size of OH masers in W3(OH). The brightness temperature inferred from our size and flux density measurements is >10¹¹K and may reach 10¹² K. Various arguments indicate that the 13.44GHz maser tends to be saturated with little line rebroadening, and that the gain coefficient is ≃3-6x10^–12m^–1 for tubes 3x10¹²m long (≃10mas). One of the most important observational results is that several narrow OH components with opposite senses of circular polarization coincide within one synthesized beamwidth. We identify these matched components with Zeeman pairs and derive the associated magnetic field strength. The field always points away from us and its strength is remarkably uniform around 10-11mG, except in the region of the arc-like structure where it is significantly weaker and ≃6-8mG. We suggest that the magnetic field plays a role in the dynamics of the OH filament and that the polarization properties of the 13.44GHz maser are consistent with the Zeeman effect and with the overlap of Zeeman components in regions with velocity differences exceeding the Zeeman splitting.