Parkes full polarization spectra of OH masers – I. Galactic longitudes 350° through the Galactic Centre to 41°

Abstract

Full polarization measurements of 1665- and 1667-MHz OH masers at sites of massive star formation have been made with the Parkes 64-m radio telescope. Here, we present the resulting spectra for 104 northerly sources, from Galactic longitude 350° through the Galactic Centre to 41°. Some maser positions were previously uncertain by many arcseconds, and thus for more than 20 masers we made new measurements with the Australia Telescope Compact Array (which also revealed several hitherto unreported masers), in most cases yielding arcsecond precision to match the majority of sites. Position improvements have assisted us in distinguishing OH masers with accompanying methanol masers from those without (thought to be at a later stage of evolution). There was no existing linear polarization information at many sites, and spectral resolution was sometimes poor, or velocity coverage incomplete. These inadequacies are addressed by the present Parkes spectra. The whole OH maser sample exhibits the well-known predominance of highly circularly polarized features. We find that linear polarization is also common, but usually much weaker, and we highlight the rare cases of very pronounced linear polarization that can extend to 100 per cent. Unusually large velocity ranges of at least 25 km s−1 are present at seven sites. Our spectra measurements for most sources are at two epochs spaced by nearly one year, and reveal high stability at most sites, and marked variability (more than factors of 2 in the strongest feature) at only five sites. The spectra also provide a valuable reference for longer term variability, with high stability evident over the past few decades at 10 sites and marked variability for four of the sample. Future systematic monitoring of these variables may uncover further examples of periodicity, a phenomenon so far recognized in only one source.