A collaborative team from Yunnan University, the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, and the University of Chile, along with other domestic and international institutions, has revealed new observational evidence that sheds light on the mystery of massive star formation. 

The findings were published in The Astrophysical Journal Supplement.

"Fragmentation Code" in Massive Star Nurseries

Stars are born within dense molecular cloud clumps in the universe, where core densities can exceed 10⁶ cm⁻³. These dense cores serve as the seeds for the formation of high-mass stars, which are more than eight times the mass of the Sun. However, how massive clumps fragment into individual star-forming core seeds remains incompletely understood. 

To address this puzzle, the researchers used the world's most powerful millimeter interferometric telescope, the Atacama Large Millimeter/submillimeter Array (ALMA), to examine a key characteristic of cloud fragmentation—the spacing between dense cores. Using ALMA, they observed 139 infrared-bright, massive protostellar clumps at a wavelength of 1.3 millimeters. Their high-resolution survey identified nearly 1,600 dense cores, showing that the average spacing between adjacent cores is nearly five times smaller than the thermal Jeans fragmentation theory predicts. 

In other words, the distribution of dense cores within the clumps is very compact, indicating that gravity dominates the fragmentation process. These new observations strongly support the idea that thermal Jeans fragmentation is the dominant mechanism in clustered high-mass star formation. 

The researchers offered two possible explanations for the compact distribution of dense cores: one resembles "Russian nesting dolls", involving multi-level fragmentation within the cloud clump; and the other involves 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.

Searching 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. 

Of the nearly 1,600 dense cores detected, the researchers identified only two massive candidates (approximately 17–21 solar masses, with a radius of about 5,000 astronomical units). Both have dense, compact structures and show only faint internal molecular line emissions, displaying no signs of known star formation activities, such as outflows. The scarcity of high-mass star-forming seed cores indicates that most high-mass stars likely 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.

This study provides compelling evidence on how high-mass stars emerge, advancing our understanding of stellar nurseries in the universe.

This study was supported by the National Key R&D Program of China through the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Yunnan Fundamental Research Projects, etc.

Detection of molecular lines inside the massive prestellar core. (Image by LIU Tie)