Maser maps and magnetic field of OH 337.705−0.053

Abstract

New high resolution studies of the Galactic maser site 337.705−0.053 reveal its magnetic field and velocity morphology. The long baseline array of the Australia Telescope National Facility provided simultaneous observations of both the 1665- and 1667-MHz OH transitions which yielded a sequence of maps at velocity spacing 0.09 km s−1, in both senses of circular polarization, with tenth-arcsec spatial resolution. 38 small diameter maser spots were detected, spread over an ellipse with largest dimension of 1.5 arcsec. Pairs of spots with the same position, but with right and left circular polarization at different frequency, reveal Zeeman splitting. Five pairs at 1665 MHz and four at 1667 MHz are seen; at one position, pairs at both transitions indicate a comparable magnetic field and similar (central) velocity. All estimates of magnetic field are in the same sense, with a median value of −2.5 mG (pointing towards the earth), confirming an interpretation from single-dish observations. The morphology and kinematics have been compared with that of maser emission from the excited state of OH at 6035 MHz, with methanol at 6668 MHz and 12 GHz, and with water at 22 GHz. All species are intermingled, and associated with an ultracompact H ii region. The site most likely lies near the tangent point of the Galactic 3-kpc ring, at a distance of 7.9 kpc. The maser spot distribution over 1.5 arcsec then corresponds to a diameter of 60 mpc, amongst the largest known, and likely to be approaching the end of the maser emitting phase. 337.705−0.053 adds to the maser sites studied in sufficient detail to explore ordered patterns in the global Galactic magnetic field.