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We report on a possible cloud-cloud collision in the DR 21 region, which wefound through molecular observations with the Nobeyama 45-m telescope. Wemapped an area of 8'x12' around the region with twenty molecular linesincluding the 12CO(J=1-0) and 13CO(J=1-0) emission lines, and sixteen of themwere significantly detected. Based on the 12CO and 13CO data, we found fivedistinct velocity components in the observed region, and we call molecular gasassociated with these components -42, -22, -3, 9, and 17 km/s clouds takingafter their typical radial velocities. The -3 km/s cloud is the mainfilamentary cloud (31,000 Mo) associated with young massive stars such as DR21and DR21(OH), and the 9 km/s cloud is a smaller cloud (3,400 Mo) which may bean 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/sclouds, and detected faint 12CO emission having intermediate velocitiesbridging the two clouds at their intersection. These facts strongly indicatethat the two clouds are colliding against each other. In addition, we foundthat DR21 and DR21(OH) are located in the periphery of the densest part of the9 km/s cloud, which is consistent with results of recent numerical simulationsof cloud-cloud collisions. We therefore suggest that the -3 and 9 km/s cloudsare colliding, and that the collision induced the massive star formation in theDR21 cloud. The interaction of the -3 and 9 km/s clouds was previouslysuggested by Dickel et al. (1978), and our results strongly support theirhypothesis of the interaction.

Cloud-cloud collision in the DR 21 cloud as a trigger of massive star formation