CO observations of the W51BHIIregion complex

We have carried out (CO)-C-12 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 g as associated with W51B is organized into a long (similar to 30' or similar to 40 pc at a distance of 5 kpc) filamentary structure approximately paral lel to the Galactic plane (b similar or equal to -18'). The filamentary str ucture appears to be composed of two separate clouds, clouds NE and SW. We derive their parameters and determined their masses to be 3.9 x 10(4) M- ., and 8.2 x 10(4) 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 o f 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 an d existing (CO)-C-13 results, we propose that the W51B clouds are externall y heated by far-ultraviolet radiation and that the (CO)-C-12 lines originat e from the photodissociation regions on the surfaces of dense (greater than or similar to 10(4) cm(-3)) clumps. We estimate the filling factors of the se clumps to be small (<0.1). Our results further suggest that the clouds d o not have low-density envelopes, which could have been destroyed by UV pho tons and stellar winds from nearby OB stars. We estimate that the star form ation efficiency (SFE) of the W51B region is very high, i.e., 7% and 15% fo r clouds NE and SW, respectively. The very high SFE could be due to enhance d star formation by the spiral density wave.