Blueshifted outflow from 24.329+0.145

Abstract

New studies of an hydroxyl (OH) maser site that has puzzled astronomers for more than 40 yr demonstrate that its emission arises in a unique strong blueshifted outflow. Measurements with the Australia Telescope Compact Array (ATCA) locate this enigmatic OH 1667-MHz maser at 24.329+0.145 (Galactic coordinates). This is coincident (to arcsecond accuracy) with maser emission at the 6668-MHz transition of methanol and a newly discovered maser at the 22-GHz transition of water. The methanol maser establishes the site as one of high-mass star formation, and the small spread of methanol features about a median velocity of +113 km s−1 defines its systemic velocity. The water maser shows a combination of some emission arising near this systemic velocity, and the remainder arising in the blueshifted outflow. The 1667-MHz OH emission extends over more than 20 km s−1, and our demonstrated association with the methanol maser reveals that this is offset from the systemic velocity by as much as 50 km s−1. The implied energetic blueshifted outflow is unprecedented in OH masers of this class. Our spectral studies using the ATCA, with a new high-resolution spectrometer, achieve full polarization observations of the 1665- and 1667-MHz OH ground-state transitions. The dominance of 1667-MHz emission, with no detection at 1665 MHz, suggests unusual physical properties in the ouflow region. Likewise, the prevalence of linear polarization may be a consequence of emission arising in a fast-moving outflow, with perhaps a link between linear polarization orientation and outflow orientation. We also present new observations of maser sites 24.494−0.039 and 24.790+0.083. These close neighbours of 24.329+0.145, with methanol maser emission at a similar systemic velocity, are also accompanied by masers of water and OH, but have no unusual maser properties. The velocity and Galactic longitude of all three sites, and of the H ii complex W42, correspond to an especially interesting Galactic location near the tangent point of the 3-kpc arm (expanding ring), at a distance of 7.7 kpc.