Massive star formation in W51A triggered by cloud-cloud collisions
  W51A is one of the most active star-forming region in our Galaxy, which
contains giant molecular clouds with a total mass of 10^6 Msun. The molecular
clouds have multiple velocity components over ~20 km/s, and interactions
between these components have been discussed as the mechanism which triggered
the massive star formation in W51A. In this paper, we report an observational
study 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 the
Nobeyama 45-m telescope at 20" resolution. Our CO data resolved the four
discrete 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 complex
G49.5-0.4, we identified four C18O clumps having sizes of ~1 pc and column
densities of higher than 10^23 cm^-3, which are each embedded within the four
velocity clouds. These four clumps are distributed close to each others within
a small distance of 5 pc, showing a complementary distribution on the sky. In
the position-velocity diagram, these clumps are connected with each others by
bridge features with intermediate intensities. The high intensity ratios of
13CO (J=3-2/J=1-0) also indicates that these four clouds are associated with
the HII regions. We also found these features in other HII regions in W51A. The
timescales of the collisions are estimated to be several 0.1 Myrs as a crossing
time of the clouds, which are consistent with the ages of the HII regions
measured from the size of the HII regions in the 21 cm continuum emissions. We
discuss the cloud-cloud collision scenario and massive star formation in W51A
by comparing with the recent observational and theoretical studies of
cloud-cloud collision.
