Rewriting stellar evolution – new results upend our understanding of the formation of high-mass stars

Astronomers have mapped 39 interstellar clouds, where high-mass stars are expected to form. These comprehensive data suggest that our current understanding of low-mass star formation may need to be expanded to take into account the formation of high-mass stars. This suggests that the formation of high-mass stars is fundamentally different from that of low-mass stars, and is not just a matter of size.

High-mass stars play an important role in the evolution of the universe by releasing heavy elements and the shock waves generated when a massive star explodes in a supernova. Despite their importance, how massive stars form remains poorly understood due to their rarity.

To better understand the formation of massive stars, a team led by Kaho Mori, Patricio Sanhueza, and Fumitaka Nakamura used the Atacama Large Millimeter/submillimeter Array (Alma) to monitor 39 infrared dark clouds (IRDCs). IRDCs are massive, cool, dense clouds of gas and dust. They are believed to be sites of massive star formation.

Dust emission maps of 39 IRDC centers where massive stars are expected to form in the future. Source: ALMA (ESO/NAOJ/NRAO), K. Morii et al.

The team focused on clouds that show no signs of star formation, to understand the beginning of the formation process before young stars ignite. In the 39 clouds, the team found more than 800 star seeds, referred to as molecular cloud cores, which astronomers believe will develop into stars.

Of these cores, 99% lack sufficient mass to become high-mass stars, assuming that high-mass stars evolve in the same way as the better-understood low-mass stars. These results support the idea that the formation mechanism of high-mass stars must be different from that of low-mass stars.

Furthermore, the team investigated the distribution of nuclei. In star clusters, high-mass stars are clustered together, while low-mass stars are widely distributed. However, this work revealed that the locations of higher-mass nuclei show no preference compared to the locations of lower-mass nuclei. On the other hand, denser nuclei tend to be locally concentrated. This suggests that denser cores, rather than more massive cores, may be the progenitors of high-mass stars; And that denser nuclei may grow more efficiently than less dense nuclei.

Reference: “ALMA Survey of 70 µm High-Mass Dark Clusters in the Early Stages (ASHES). IX. “Physical Properties and Spatial Distribution of Nuclei in IRDCs” by Kaho Mori, Patrick Sanhueza, Fumitaka Nakamura, Zhichu Zhang, Giovanni Sabatini, Henrik Böther, Xing Lu, Shanggu Li, Guido Garay, James M. Jackson, Fernando A. Olguin, Daniel Tafoya, Kenichi Tatematsu, Natsuko Izumi, Takeshi Sakai and Andrea Silva, June 20, 2023, the Astrophysical Journal.
DOI: 10.3847/1538-4357/accea

The study was funded by the Japan Society for the Promotion of Science, the German Research Foundation, and the National Research and Development Agency.