Molecular clouds toward three Spitzer bubbles S116, S117 and S118:
  Evidence for the cloud-cloud collision which formed the three \HII \ regions
  and a 10-pc scale molecular cavity
  We carried out a molecular line study toward the three Spitzer bubbles S116,
S117 and S118 which show active formation of high-mass stars. We found
molecular gas consisting of two components with velocity difference of {$\sim
5$ \kms}. One of them, the small cloud, has typical velocity of {$-63$ \kms} \
and the other, the large cloud, has that of $-58$ \kms. The large cloud has a
nearly circular intensity depression whose size is similar to the small cloud.
We present an interpretation that the cavity was created by a collision between
the two clouds and the collision compressed the gas into a dense layer
elongated along the western rim of the small cloud. In this scenario, the O
stars including those in the three Spitzer bubbles were formed in the interface
layer compressed by the collision. By assuming that the relative motion of the
clouds has a tilt of \timeform{45D} to the line of sight, we estimate that the
collision continued over the last 1 Myr at relative velocity of $\sim$10 \kms.
In the S116--117--118 system the \HII \ regions are located outside of the
cavity. This morphology is ascribed to the density-bound distribution of the
large cloud which made the \HII \ regions more easily expand toward the outer
part of the large cloud than inside of the cavity. The present case proves that
a cloud-cloud collision creates a cavity without an action of O star feedback,
and suggests that the collision-compressed layer is highly filamentary.
