# 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

## TL;DR

This study presents evidence that cloud-cloud collisions in molecular clouds associated with three Spitzer bubbles triggered high-mass star formation and created a molecular cavity, highlighting a collision-driven star formation mechanism.

## Contribution

It provides observational evidence that cloud-cloud collisions can form cavities and trigger high-mass star formation without stellar feedback, emphasizing the role of collisions in molecular cloud evolution.

## Key findings

- Identified two molecular cloud components with a 5 km/s velocity difference.
- Detected a cavity in the large cloud likely formed by cloud collision.
- Estimated collision duration of about 1 million years at 10 km/s.

## Abstract

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.