VLBA POLARIZATION OBSERVATIONS OF THE J=7 2, 13.44 GHZ OH MASER IN W3(OH)/

We have obtained the first polarization images of the J = 7/2, F = 4-4 OH maser emission from W3(OH) at 13.44 GHz with high dynamic range an d resolution. The emission has been imaged with a spectral resolution of 0.087 km s(-1) and with a spatial resolution of 670 mu arcsec, the highest ever achieved in any image of the OH radical. The 13.44 GHz OH maser emission lies within an extremely compact area of 75 x 105 mill iarc sec (mas) centered on the edge of the northern ionized clump obse rved in high frequency continuum images of W3(OH). We determined the a bsolute position of the -42.89 km s(-1) reference feature to an accura cy of 0.'' 01-0.'' 02 thus allowing us to register OH images taken at different epochs. The area of OH emission contains spatially and kinem atically distinct subregions. Among them, we discovered a remarkable f ilament extending over approximate to 15 mas (or 30 AU). This filament is part of a larger scale are structure nearly 60 AU in size, the fir st arc-like structure ever observed in OH masers. A high velocity grad ient and a magnetic field gradient are observed along this are. Most o f the OH flux density is contained at a position lying nearly at the c enter of the are structure, and perhaps marking the site of the obscur ed O-type star that excites W3(OH). Our data show that elongated OH-em itting structures, perhaps related to local shocks, are dominant, and that no simple geometry exists for the individual OH maser spots. Howe ver, the observations made with the longest baselines are consistent w ith a size of about 0.4 mas (or approximate to 0.9 AU). This is the mo st compact size of OH masers in W3(OH). The brightness temperature inf erred from our size and flux density measurements is >10(11) K and may reach 10(12) K. Various arguments indicate that the 13.44 GHz maser t ends to be saturated with little line rebroadening, and that the gain coefficient is approximate to 3-6 10(-12) m(-1) for tubes 3 10(12) m l ong (approximate to 10 mas). One of the most important observational r esults is that several narrow OH components with opposite senses of ci rcular polarization coincide within one synthesized beamwidth. We iden tify these matched components with Zeeman pairs and derive the associa ted magnetic field strength. The field always points away from us and its strength is remarkably uniform around 10-11 mG, except in the regi on of the are-like structure where it is significantly weaker and appr oximate to 6-8 mG. We suggest that the magnetic field plays a role in the dynamics of the OH filament and that the polarization properties o f the 13.44 GHz maser are consistent with the Zeeman effect and with t he overlap of Zeeman components in regions with velocity differences e xceeding the Zeeman splitting.