1999ApJ...518..760K

We have carried out ¹²CO J=1-0 and J=2-1 line observations of the W51B H II region complex. CO J=1-0 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≃-18'). The filamentary structure appears to be composed of two separate clouds, clouds NE and SW. We derive their parameters and determined their masses to be 3.9x10⁴ M_☉ and 8.2x10⁴ M_☉, respectively. The compact H II region G49.2-0.3 is associated with cloud NE and all the other compact H II regions in W51B are associated with cloud SW.

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 nearby molecular clouds. Based on the large observed ratios and existing ¹³CO results, 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 (≳10⁴ cm^–3) clumps. We estimate the filling factors of these clumps to be small (<0.1). Our results further suggest that the clouds do not have low-density envelopes, which could have been destroyed by UV photons and stellar winds from nearby OB stars. We estimate that the star formation efficiency (SFE) of the W51B region is very high, i.e., 7% and 15% for clouds NE and SW, respectively. The very high SFE could be due to enhanced star formation by the spiral density wave.