A kinematic analysis of the CO clouds toward a reflection nebula NGC 2023 observed with the Nobeyama 45 m telescope; Further evidence for a cloud-cloud collision in the Orion region

TL;DR

This study provides high-resolution evidence of a cloud-cloud collision in the Orion region, which likely triggered the formation of a high-mass star and multiple lower-mass stars, challenging previous HII region triggered star formation models.

Contribution

It offers new high-resolution kinematic data supporting cloud-cloud collision as the main star formation mechanism in Orion, contrasting with prior HII region triggered models.

Findings

Detection of two velocity components indicating cloud interaction

Evidence that cloud collision triggered high-mass star formation

Limited influence of HII region on gas compression

Abstract

We have analyzed new CO($J$ = 1-0) data in the region of a reflection nebula NGC 2023 with a particular focus on the detailed kinematical properties of the molecular gas. The results show that there are two velocity components which indicate signatures of dynamical interaction revealed at a high resolution of 19$''$ (= 0.04 pc). Based on the results we propose a hypothesis that two clouds collided with each other and triggered the formation of the B1.5 star HD 37903 in addition to 20 lower mass stars in two small clusters with a size of 2 pc. Although the previous study favored a scheme of triggering by the HII region (e.g., Mookerjea et al. 2009), the present results show that the effect of the HII region is limited only to the surface of the molecular cloud, and does not contribute to the gas compression and star formation. The present results lend support for the dominant role of cloud-cloud collision in forming high mass stars in addition to $\sim$20 lower mass stars, which are also likely formed by the collision. The present case suggests all the high mass stars in the Orion region are formed by cloud-cloud collision.