Cloud-cloud collision in the DR 21 cloud as a trigger of massive star
  formation
  We report on a possible cloud-cloud collision in the DR 21 region, which we
found through molecular observations with the Nobeyama 45-m telescope. We
mapped an area of 8'x12' around the region with twenty molecular lines
including the 12CO(J=1-0) and 13CO(J=1-0) emission lines, and sixteen of them
were significantly detected. Based on the 12CO and 13CO data, we found five
distinct velocity components in the observed region, and we call molecular gas
associated with these components -42, -22, -3, 9, and 17 km/s clouds taking
after their typical radial velocities. The -3 km/s cloud is the main
filamentary cloud (31,000 Mo) associated with young massive stars such as DR21
and DR21(OH), and the 9 km/s cloud is a smaller cloud (3,400 Mo) which may be
an extension of the W75 region in the north. The other clouds are much smaller.
We found a clear anticorrelation in the distributions of the -3 and 9 km/s
clouds, and detected faint 12CO emission having intermediate velocities
bridging the two clouds at their intersection. These facts strongly indicate
that the two clouds are colliding against each other. In addition, we found
that DR21 and DR21(OH) are located in the periphery of the densest part of the
9 km/s cloud, which is consistent with results of recent numerical simulations
of cloud-cloud collisions. We therefore suggest that the -3 and 9 km/s clouds
are colliding, and that the collision induced the massive star formation in the
DR21 cloud. The interaction of the -3 and 9 km/s clouds was previously
suggested by Dickel et al. (1978), and our results strongly support their
hypothesis of the interaction.
