2009A&A...494..117S

While there is evidence for considerable circular polarization of OH masers in star-forming regions, no systematic measurements of the four Stokes parameters have been done. Determining the full polarization properties of OH emission is vital to test polarization maser models and to select targets for high-angular resolution studies of magnetic fields. The polarimetric measurements of a representative sample of star-forming regions were carried out in order to better characterize the properties of OH masers. A sample of 99 star-forming regions was observed in the OH 1665 and 1667 MHz maser transitions using the Nancay radio telescope in full-polarization mode and high spectral resolution (0.07km/s). We present a complete set of polarization spectra for all 98 and 81 sources detected at 1665 and 1667MHz, respectively. All detections have circularly polarized features, whereas linear polarization is found in 80% and 62% of the 1665 and 1667MHz sources, respectively. The median degree of linear polarization of 16% of both transitions is a factor of ∼4 lower than that of circular polarization. In consequence, the probability of linear polarization clearly drops for sources with low total flux density because of the sensitivity limit. The linear polarization is almost always accompanied by circular polarization, i.e., elliptically polarized profiles are observed. The mean line-width at half-maximum of the linear polarization profile is 0.26km/s. The polarization position angle (χ) shows large variations across the profile with a median amplitude close to 90°. The amplitude variations of χ decrease with increasing flux density. Large variations of the total flux density found in three sources are weakly associated with changes in polarization parameters. The observational characteristics of linear polarization seem to be consistent with theoretical models where narrow profiles result from the overlap of σ-components traversed regions with appreciable velocity gradient and due to off resonance amplification.