Galactic methanol masers at 6.6 GHz

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Abstract
We have searched for 6.6-GHz methanol maser emission from the directions of more than 200 OH masers, together with a number of other sites of star formation. We have also observed previously reported methanol masers south of declination 24°. Where we detected methanol emission, we measured its position and have found more than 40 instances of small clusters of sources. We present observations of a total of 245 methanol masers with peak intensities ranging from 0.3 to 5000 Jy. For a typical source, the velocity range of the emission is quite large - commonly 10 km s-1 and, in a few instances, exceeding 20 km s-1. Some of the largest velocity ranges are suggestive of emission from an outflow region as well as a central source. Our observtions over several epochs show that many sources possess at least some features whose intensity varies on a time-scale of several months. Towards most of the 6.6-GHz masers we have searched for 12-GHz methanol maser emission, and have made 131 detections; the median ratio of 12-to 6.6-GHz intensity if 0.11. We discuss the general properties of this large sample of 6.6-GHz methanol masers. They appear to be more prevalent than OH masers, with intensities typically seven times higher, and are thus excellent tracers of compact H II regions and sites of recent star formation.

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