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Paper: On-the-Fly Imaging of the W51B Complex
Volume: 217, Imaging at Radio Through Submillimeter Wavelengths
Page: 88
Authors: Koo, B.-C.
Abstract: W51B is part of the radio complex W51, which is one of the most active star-forming regions in the Galaxy. W51B sources are associated with a stream of atomic and molecular gases, known as the ``high-velocity (HV) stream". The HV stream is considered to be the gas streaming along the Sagittarius spiral arm resulting from the perturbation due to the spiral potential. We have carried out 12CO J = 1--0 and J = 2--1 line observations of W51B using the NRAO 12m telescope. The observations show that the molecular gas associated with W51B is organized into a long (~30' or ~40 pc at a distance of 5 kpc), filamentary structure approximately parallel to the Galactic plane (b~eq -18'). The filamentary structure appears to be composed of two separate clouds. We derive CO J = 2--1/J = 1--0 luminosity ratios of 1.2 and 1.4 for these clouds, which are much higher than typical ratios (0.5--0.8) for neaby molecular clouds (Sakamoto et al. 1997). We propose that the W51B clouds are externally heated by far-ultraviolet radiation and that the 12CO lines originate from the photodissociation regions on the surfaces of dense (~gt 104 cm-3) clumps. We estimate the filling factors of these clumps to be small (< 0.1). The star-formation efficiency (SFE) of the W51B region is very high (7% and 15%). The very high SFE could be due to enhanced star formation by the spiral density wave.
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