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W51A is one of the most active star-forming region in our Galaxy, whichcontains giant molecular clouds with a total mass of 10^6 Msun. The molecularclouds have multiple velocity components over ~20 km/s, and interactionsbetween these components have been discussed as the mechanism which triggeredthe massive star formation in W51A. In this paper, we report an observationalstudy of the molecular clouds in W51A using the new 12CO, 13CO, and C18O(J=1-0) data covering a 1.4x1.0 degree region of W51A obtained with theNobeyama 45-m telescope at 20" resolution. Our CO data resolved the fourdiscrete velocity clouds at 50, 56, 60, and 68 km/s with sizes and masses of~30 pc and 1.0-1.9x10^5 Msun. Toward the central part of the HII region complexG49.5-0.4, we identified four C18O clumps having sizes of ~1 pc and columndensities of higher than 10^23 cm^-3, which are each embedded within the fourvelocity clouds. These four clumps are distributed close to each others withina small distance of 5 pc, showing a complementary distribution on the sky. Inthe position-velocity diagram, these clumps are connected with each others bybridge features with intermediate intensities. The high intensity ratios of13CO (J=3-2/J=1-0) also indicates that these four clouds are associated withthe HII regions. We also found these features in other HII regions in W51A. Thetimescales of the collisions are estimated to be several 0.1 Myrs as a crossingtime of the clouds, which are consistent with the ages of the HII regionsmeasured from the size of the HII regions in the 21 cm continuum emissions. Wediscuss the cloud-cloud collision scenario and massive star formation in W51Aby comparing with the recent observational and theoretical studies ofcloud-cloud collision.

Massive star formation in W51A triggered by cloud-cloud collisions