High-mass star formation in Orion B triggered by cloud-cloud collision: Merging molecular clouds in NGC 2024

TL;DR

This study presents evidence that high-mass star formation in Orion B's NGC 2024 region was triggered by a cloud-cloud collision, supported by new $^{13}$CO observations revealing colliding molecular clouds.

Contribution

It provides observational evidence linking cloud-cloud collision to high-mass star formation in Orion B, highlighting the role of such collisions in triggering star formation.

Findings

Two independent velocity components are physically connected to the HII region.

Complementary cloud distribution suggests a collision with a 0.6 pc displacement.

Collision duration estimated at 0.3 million years.

Abstract

We performed new comprehensive $^{13}$CO($J$=2--1) observations toward NGC 2024, the most active star forming region in Orion B, with an angular resolution of $\sim$100'' obtained with NANTEN2. We found that the associated cloud consists of two independent velocity components. The components are physically connected to the H{\sc ii} region as evidenced by their close correlation with the dark lanes and the emission nebulosity. The two components show complementary distribution with a displacement of $\sim$0.6 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that a cloud-cloud collision between the two components triggered the formation of the late O-type stars and early B stars localized within 0.3 pc of the cloud peak. The duration time of the collision is estimated to be 0.3 million years from a ratio of the displacement and the relative velocity $\sim$3 km s$^{-1}$ corrected for probable projection. The high column density of the colliding cloud $\sim$10$^{23}$ cm$^{-2}$ is similar to those in the other high-mass star clusters in RCW 38, Westerlund 2, NGC 3603, and M42, which are likely formed under trigger by cloud-cloud collision. The present results provide an additional piece of evidence favorable to high-mass star formation by a major cloud-cloud collision in Orion.