A research team from the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences has conducted a systematic study of protocluster samples using data from the ALMA-QUARKS survey (Atacama Large Millimeter/submillimeter Array – Querying Underlying mechanisms of massive star formation with ALMA-Resolved gas Kinematics and Structures). The study reveals that massive hot cores can emerge at various evolutionary stages of high-mass protostars, challenging the long-held view that hot cores represent only a short-lived phase in stellar evolution.

The study was recently published in The Astrophysical Journal.

Leveraging high-resolution, high-sensitivity 1.3 mm continuum and molecular line observations from ALMA, the researchers identified 125 hot molecular fragments. These fragments correspond to the substructures of hot cores, as revealed at high spatial resolution.

Based on the fragments' associations with CO molecular outflows and H II regions, the team classified them into four categories: those linked to narrow-angle outflows, wide-angle outflows, no outflows, and shell-like structures surrounding H II regions.

This diversity suggests that hot cores are not confined to a specific evolutionary stage. Instead, they persist throughout the formation of high-mass stars and exhibit a range of morphologies.

By analyzing hot molecular lines and hydrogen recombination lines, the researchers examined the spatial relationship between hot molecular fragments and ultra-compact H II regions. They found a clear spatial separation between the two, indicating that the sequential formation of massive stars within young protoclusters is unlikely to be directly triggered by feedback from pre-existing H II regions. This result provides new observational constraints on the role of feedback mechanisms in high-mass star formation.

Drawing on a large statistical sample, the study uncovers the diverse evolutionary manifestations of hot cores, revising and enriching scientific understanding of the evolutionary stages of high-mass star formation. It also holds implications for the development and refinement of theoretical models describing massive star formation.