2010ApJS..190...58K

We present the results of an extensive observational study of the active star-forming complex W51 that was observed in the J = 2 - 1 transition of the ¹²CO and ¹³CO molecules over a 1{fdg}25x1{fdg}00 region with the University of Arizona Heinrich Hertz Submillimeter Telescope. We use a statistical equilibrium code to estimate physical properties of the molecular gas. We compare the molecular cloud morphology with the distribution of infrared (IR) and radio continuum sources and find associations between molecular clouds and young stellar objects (YSOs) listed in Spitzer IR catalogs. The ratios of CO lines associated with H II regions are different from the ratios outside the active star-forming regions. We present evidence of star formation triggered by the expansion of the H II regions and by cloud-cloud collisions. We estimate that about 1% of the cloud mass is currently in YSOs.