Observations of high-mass star seeds defy the model

June 20, 2023

(Nanowerk News) Astronomers have mapped 39 interstellar clouds where high-mass stars are expected to form. This large data set indicates that the accepted model of low-mass star formation needs to be expanded to account for high-mass star formation. This shows that the formation of high-mass stars is fundamentally different from the formation of low-mass stars, it’s not just a matter of scale.

The research has been published in Astrophysics Journal (“ALMA Survey of 70 µm Dark High-mass Clots at Early Stage (ASHES). IX. Physical Properties and Spatial Distribution of Cores in IRDC”). Dust emission map for 39 IRDCs where massive stars are expected to form in the future. (Image: ALMA (ESO/NAOJ/NRAO), K. Morii et al)

High-mass stars play an important role in the evolution of the universe through the release of heavy elements and the shock waves generated when a massive star explodes in a supernova. Despite their importance, how massive stars form is still poorly understood because of their rarity.

To better understand massive star formation, the team led by Kaho Morii, Patricio Sanhueza, and Fumitaka Nakamura used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe 39 infrared dark clouds (IRDC). IRDC is a large, cold, dense cloud of gas and dust; and is thought to be the site of massive star formation. The team focused on clouds showing no signs of star formation, to understand the beginning of the formation process before young stars flare. In the 39 clouds, the team found more than 800 stellar seeds, known as molecular cloud cores, which astronomers say will evolve into stars.

Of this core, 99% lacks the mass to be a high-mass star, assuming that high-mass stars evolve in the same way low-mass stars are better understood. This finding supports the idea that the mechanism of formation of high-mass stars must be different from that of low-mass stars.

Next, the team investigated the core distribution. In star clusters, high-mass stars are grouped together, while low-mass stars are widely distributed. However, this work reveals that higher-mass core locations show no preference over lower-mass core positions. On the other hand, denser nuclei tend to be locally concentrated. This suggests that denser rather than more massive nuclei may be the ancestors of high-mass stars; and a denser core can grow more efficiently than a less dense core.

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