Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011mnras.411.2059c&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 411, Issue 3, pp. 2059-2066.
Astronomy and Astrophysics
Astronomy
2
Masers, Stars: Formation, H Ii Regions, Ism: Molecules, Galaxy: Kinematics And Dynamics, Radio Lines: Ism
Scientific paper
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.
Caswell J. L.
Green Andrew J.
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