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We have analyzed new CO($J$ = 1-0) data in the region of a reflection nebulaNGC 2023 with a particular focus on the detailed kinematical properties of themolecular gas. The results show that there are two velocity components whichindicate signatures of dynamical interaction revealed at a high resolution of19$''$ (= 0.04 pc). Based on the results we propose a hypothesis that twoclouds collided with each other and triggered the formation of the B1.5 star HD37903 in addition to 20 lower mass stars in two small clusters with a size of 2pc. Although the previous study favored a scheme of triggering by the HIIregion (e.g., Mookerjea et al. 2009), the present results show that the effectof the HII region is limited only to the surface of the molecular cloud, anddoes not contribute to the gas compression and star formation. The presentresults lend support for the dominant role of cloud-cloud collision in forminghigh mass stars in addition to $\sim$20 lower mass stars, which are also likelyformed by the collision. The present case suggests all the high mass stars inthe Orion region are formed by cloud-cloud collision.

publications of the astronomical society of japan

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