The formation of a Spitzer bubble RCW79 triggered by cloud-cloud collision

TL;DR

This study suggests that the formation of multiple O stars in the RCW79 bubble was triggered by a cloud-cloud collision, supported by molecular emission observations and collision signatures, occurring over about 1 million years.

Contribution

It provides observational evidence that cloud-cloud collision can trigger massive star formation and explains the large size of RCW79 compared to similar regions.

Findings

Molecular clouds in RCW79 are associated with four velocity components.

Collision signatures support the cloud-cloud collision hypothesis.

The collision process occurred over approximately 1 million years.

Abstract

Understanding the mechanism of O star formation is one of the most important issues in current astrophysics. It is also an issue of keen interest how O stars affect their surroundings and trigger secondary star formation. An H\,\emissiontype{II} region RCW79 is one of the typical Spitzer bubbles alongside of RCW120. New observations of CO $J=$ 1--0 emission with Mopra and NANTEN2 revealed that molecular clouds are associated with RCW79 in four velocity components over a velocity range of 20 km s$^{-1}$. We hypothesize that two of the clouds collided with each other and the collision triggered the formation of 12 O stars inside of the bubble and the formation of 54 low mass young stellar objects along the bubble wall. The collision is supported by observational signatures of bridges connecting different velocity components in the colliding clouds. The whole collision process happened in a timescale of $\sim$1 Myr. RCW79 has a larger size by a factor of 30 in the projected area than RCW120 with a single O star, and the large size favored formation of the 12 O stars due to the larger accumulated gas in the collisional shock compression.