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.
Download