High-mass star formation in Orion triggered by cloud-cloud collision II, Two merging molecular clouds in NGC2024

TL;DR

This study provides evidence that high-mass star formation in NGC 2024 is likely triggered by a collision between two molecular clouds, supported by observational data and analysis of cloud dynamics.

Contribution

It demonstrates the role of cloud-cloud collision in triggering high-mass star formation in NGC 2024, with detailed analysis of molecular cloud components.

Findings

Two velocity components are physically connected to the HII region.

Complementary distribution of clouds suggests collision.

Collision timescale estimated at ~10^5 years.

Abstract

We analyzed the NANTEN2 13CO (J=2-1 and 1-0) datasets in NGC 2024. We found that the cloud consists of two velocity components, whereas the cloud shows mostly single-peaked CO profiles. The two components are physically connected to the HII 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 0.4 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that cloud-cloud collision between the two components triggered the formation of the late O stars and early B stars localized within 0.3 pc of the cloud peak. The collision timescale is estimated to be ~ 10^5 yrs from a ratio of the displacement and the relative velocity 3-4 km s-1 corrected for probable projection. The high column density of the colliding cloud 1023 cm-2 is similar to those in the other massive 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.