Recently, a collaborative team from Yunnan University, Shanghai Astronomical Observatory, Chinese Academy of Sciences, and the University of Chile, among other domestic and international institutions, has revealed new observational evidence unraveling the mystery of massive star birth. The related findings have been published in the international academic journal The Astrophysical Journal Supplement.

"Fragmentation Code" in Massive Stars’ Nurseries

Stars are born within dense molecular cloud clumps in the universe (e.g., with densities greater than 10⁴ cm⁻³). These clumps contain dense molecular cores (e.g., with densities higher than 10⁶ cm⁻³) that serve as seeds for the formation of high-mass stars (exceeding 8 times the solar mass). However, how massive clumps fragment into individual star-forming core seeds remains incompletely understood. To address this puzzle, the research team employed the world’s most powerful millimeter interferometric telescope, ALMA, aiming to scrutinize a key characteristics of cloud fragmentation—the spacing between dense cores. Using the ALMA telescope, the team conducted high angular-resolution observations of 139 infrared-bright massive protostellar clumps at a wavelength of 1.3 mm, carefully identifying nearly 1,600 dense cores. The statistically significant results show that the average spacing between adjacent cores is nearly five times smaller than predicted by the thermal Jeans fragmentation theory. In other words, the distribution of dense cores within the clumps is very compact, indicating that the driving mechanism of the fragmentation process is dominated by gravity. This new observational result strongly elucidates the fragmentation mechanism in clustered high-mass star formation—thermal Jeans fragmentation. Furthermore, the researchers suggest that the compact distribution of dense cores is associated with two most probable physical processes: one resembles “Russian nesting dolls”, involving multi-level fragmentation within the cloud clump; or, due to the dynamical evolution of the natal clump, newly formed cores become increasingly crowded over time. Further observational verification of these two possibilities could provide key constraints for existing theoretical models of star formation.

Figure 1: (Left) Example of a massive protostellar cloud clump. (Right) High angular-resolution ALMA observation image of the clump, with plus signs marking the positions of dense cores; the spacing between adjacent cores is indicated by green line segments.

Search for Massive Star-Forming Seeds—A Key Decision in the "Growth Path"

Another highlight of the study is the search for high-mass star-forming seeds—massive starless cores. They are supposed to be massive (at least exceeding 16 solar masses), extremely dense (>10⁶ cm⁻³), and have not yet begun any star formation activity. Such cores serve as critical criteria for determining the formation pathway of high-mass stars. For instance, the prevailing "turbulent core accretion" model posits that high-mass stars form from pre-existing, isolated starless cores through gravitational collapse and rapid accretion of surrounding material. In contrast, the widely discussed "competitive accretion" model suggests that high-mass stars originate from a cluster of low-mass cores that grow into "big ones" through competitive accretion for gaseous material. Among the nearly 1,600 detected dense cores, the researchers identified only two massive candidates (approximately 17–21 solar masses, with a radius of about 5,000 astronomical units). Both present dense and compact density structures (e.g., Figure 2), show only faint molecular line emissions internally (Figure 3), and display no signs of known star formation activities such as outflows. This scarcity of high-mass star-forming seed cores indicates that most high-mass stars probably grow from a cluster of low-mass cores through intense competition and continuous accretion of material, providing strong new observational evidence in support of the "competitive a ccretion" model.

Figure 2: Continuum image of a candidate massive prestellar core.

Figure 3: Detection of molecular lines inside the massive prestellar core.

This research received funding from various sources, including the National Key R&D Program of China through the Ministry of Science and Technology, the National Natural Science Foundation of China, the Yunnan Fundamental Research Projects, the Yunnan "Xingdian Talent Support Plan" Youth Project, the Strategic Priority Research Program of the Chinese Academy of Sciences, the Shanghai Natural Science Foundation, and the "Light of West China" Program of the Chinese Academy of Sciences.

Doi:https://iopscience.iop.org/article/10.3847/1538-4365/adf847

Scientific Contact:

Liu Tie, liutie@shao.ac.cn ;

Hong-Li Liu, hongliliu2012@gmail.com