Magnetic fields from OH maser maps at 6035 and 6030 MHz at Galactic sites 351.417+0.645 and 353.410−0.360

Abstract

Hydroxyl (OH) masers at two Galactic sites of massive star formation have been studied using the Long Baseline Array of the Australia Telescope National Facility. The 6035- and 6030-MHz OH excited-state transitions were observed, yielding a series of maps at velocity spacing 0.10 km s−1, in both senses of circular polarization, with angular resolution of approximately 50 mas. Within a radius of several arcsec each site displays many maser spots. Pairs of spots with the same position, but with right and left circular polarization separated in frequency, reveal Zeeman splitting. Towards 351.417+0.645, positions and velocities were measured for 56 discrete maser spots. Remarkably, all of these are components of Zeeman pairs, with 6035-MHz pairs at 23 distinct locations, five of which show matching 6030-MHz pairs. Strikingly, at 14 southerly locations the magnetic field is typically −5 mG (towards us), whereas seven of the northerly locations show a magnetic field of opposite sign, of up to +6.4 mG. The velocity field spanning −11.2 to −5 km s−1 shows no simple pattern. Maser emission towards 353.410−0.360 is confined to half an arcsecond total extent, comprising 24 maser spots, most of them members of nine Zeeman pairs. Derived magnetic fields are all negative, three between −4.8 and −10.6 mG and three much weaker fields between 0 and −1.6 mG. There is no simple pattern in the velocity field. OH masers at the 1665- and 1720-MHz transitions, and prominent ultracompact H ii regions, are present at both 351.417+0.645 and 353.410−0.360. They are accompanied by methanol maser emission at the 6668-MHz, 12-GHz, and 107-GHz transitions. Masers at the 1667-MHz transition of OH and at the 22-GHz transition of water are found towards 351.417+0.645, but not towards 353.410−0.360. The magnetic fields of both sites are in accord with tracing an ordered Galactic magnetic field.