Scientists have found new evidence that cloud-cloud collisions lead to massive star formation

Massive star formation is one of the hot and difficult problems in today’s sotrophysics research, and the two major models of massive star formation (competitive accretion and mononuclear collapse) have certain deficiencies. Competitive accretion does not explain isolated massive star formation well, while single, independent OB stars are common in galaxies. The single-core collapse model does not explain how dense clumps (nuclei) with column densities greater than 1 gcm-2 are generated. If the timescale for the formation of massive, high-density clumps (nuclei) is comparable to the timescale for the free-falling of low-mass stars, the low-mass stars will form first, and their feedback will hinder the formation of massive stars. Therefore, the time for large-mass, high-density clumps (nuclei) to form in a single-core collapse model must be short. Fast external compression provides a mechanism for the rapid formation of large-mass, high-density clumps (nuclei) (column densities greater than 1 gcm-2).

There are molecular clouds of large amounts of molecular gas widely distributed in galaxies, and the “cloud collisions” between them are typical examples of rapid external compression. Recently, Ma Yingxiu, a doctoral student in the Star Formation and Evolution Group of the Xinjiang Observatory, and collaborators used multi-band continuous spectral data and 12CO and 13CO spectral line data to study the molecular cloud complex 323.18+0.15, and found strong evidence that cloud-cloud collisions led to massive star formation. The results of this research have been published in Astronomy and Astrophysics (2022, A&A, 663, A97).

Fig. 1 Morphological and spectral line characteristics of collision molecular clouds

The researchers found that G323.18+0.15 contained three different velocity components, G323.18a, G323.18b, and G323.18c. G323.18b presents a perfect U-shaped structure (top left corner of Figure 1), which is the morphological structure that a smaller molecular cloud typically leaves behind when crashing into a larger molecular cloud. G323.18a complements the U-shaped structure G323.18b morphologically, indicating that G323.18a collides with G323.18b (Figure 1). The position-velocity clearly shows the dense composition (Bridge structure) formed by their collision and mixing. In addition, the analysis of the gravitational equilibrium state of molecular clouds and the distribution of young stars also proved that cloud-cloud collisions occurred in the molecular cloud complex G323.18+0.15. (Source: Xinjiang Astronomical Observatory, Chinese Academy of Sciences)

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