A new research published in Science China Physics, Mechanics & Astronomy shows that turbulence, rather than gravity, may be the primary driver governing gas kinematics and structural evolution even within sub-parsec supercritical gas filaments near nascent stars. This finding delivers critical observational evidence for deciphering the dynamical mechanisms active during the initial stage of star formation.

Led by Chinese scholars in collaboration with scientists from Finland, the United States, Spain, Chile, Germany, South Korea and other countries, this research—based on data from the ALMA-ATOMS survey—challenges the long-standing canonical paradigm in astronomy that gravity dominates the small-scale structural evolution of molecular clouds.

Stars form within dense gas filamentary structures found in molecular clouds. The fragmentation, collapse, and mass accretion of such filaments remain a core research frontier in contemporary astrophysics. Two dominant theoretical frameworks have long prevailed: one posits that supersonic turbulence shapes and fragments filamentary structures, while the other holds that gravitational forces act as the key regulator.

Previous observational efforts have mostly focused on large-scale filaments spanning 1–10 parsecs and their bulk motions. However, systematic statistical dynamical investigations targeting sub-parsec regions (<1 parsec)—the core cradle of star birth—have been scarce, leaving the two competing theoretical models inadequately constrained by observational data.

To address this gap, the research team analyzed high-sensitivity H¹³CO⁺ J=1–0 molecular line data from the Atacama Large Millimeter/submillimeter Array (ALMA) ATOMS survey across 146 active massive star-forming regions in the Milky Way. The optically thin H¹³CO⁺ J=1–0 line enables precise tracing of dense gas kinematics.

Artist's impression: Gas moves chaotically on small scales in massive star-forming regions. (Image by SHAO)

Using the CRISPy algorithm, the researchers identified 837 kinematically coherent gas filaments within position-position-velocity (PPV) 3D space, of which 214 have an aspect ratio greater than 5. The filament lengths range from 0.02 pc to 1.6 pc, with a median length of 0.23 pc. Statistical analysis further shows that 98% of the sample (823 filaments) possess line densities exceeding the critical threshold, classifying them as gravitationally bound supercritical filaments. Conventional theories predict such structures would undergo ordered gravitational contraction and fragmentation to spawn stars.

To probe deeper, the team pioneered a vector field decomposition technique to resolve pixel-by-pixel distributions of internal velocity gradients, intensity gradients, and gravitational fields within filaments. All gradients were decomposed into two orthogonal components parallel and perpendicular to the filament major axis, disentangling radial motions along filaments from transverse cross-filament motions. Observational measurements reveal that the magnitudes of parallel and local perpendicular velocity gradients inside filaments are nearly equivalent, indicating gas undergoes substantial transverse motions alongside radial streaming along filaments.

Further orientation analysis yielded paradigm-shifting results. The team quantified the angular distributions of velocity gradients, density intensity gradients, and gravitational fields relative to filament major axes: density and gravitational gradients show strong preferential alignment perpendicular to filament axes, whereas velocity gradients display fully random spatial orientations with no correlation to filament axes or local gravitational fields. Chaotic gas motions persist even down to sub-0.1 pc scales, domains widely presumed to be gravitationally dominated. Correlation tests confirm no significant coupling between velocity gradients and gas surface density or local gravitational acceleration, effectively ruling out local gravity as the dominant driver of gas dynamics.

Professor ZHANG Chao from Taiyuan Normal University, first author of the paper, commented: "We initially set out to characterize ordered, regular gas accretion flows within dense filamentary structures via velocity gradient analysis, yet were astonished to discover highly chaotic gas motions inside these high-density filaments."

The team compared observational results against numerical simulations of randomly driven supersonic magnetohydrodynamic (MHD) turbulence. Synthetic observables generated by simulations closely reproduce key observational signatures including velocity gradient distributions and kinematic modes, convincingly demonstrating that unordered internal gas motions arise from isotropic turbulence. Minor discrepancies persist between simulations and observations: simulated transverse velocity gradients marginally exceed radial counterparts, and velocity gradients strengthen with rising gas surface density—a trend absent in observational datasets.

Researchers attribute these disparities to the extreme turbulence intensity of the observed active massive star-forming regions, which far exceeds that of low-mass star-forming regions targeted in simulations, alongside divergent magnetic field configurations and gas density regimes between simulation and observation. These mismatches highlight clear directions for refined numerical modeling in future work.

Dr. LIU Tie, Principal Investigator of the ATOMS project and corresponding author based at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS), stated: "This work overturns the traditional view that gravity governs small-scale molecular cloud structure formation, offering a novel framework to reconcile long-standing tensions between turbulent and gravitational models of star formation. Moving forward, our team will integrate high-precision numerical simulations and multi-wavelength observations to further unravel coupled interactions among turbulence, gravity, and magnetic fields, gradually unveiling the full evolutionary sequence of star formation."

Professor Philippe André, an expert on molecular cloud filamentary structures at Paris-Saclay University, remarked in a perspective article: "This is a startling new result since HFS clumps are strongly self-gravitating systems in which gravity has often been considered to be the main dynamical player, akin to the case of clusters of galaxies at the nodes of the cosmic web. If confirmed, such a conclusion would have profound implications as it would significantly reshape our – admittedly limited– understanding of the initial formation phase(s) of star clusters and massive stars."

Theoretical astrophysicist Professor Mark R. Krumholz from the Australian National University said: "This finding also eases a major tension that had arisen between the picture of star formation painted by high-resolution observations of particular star-forming clouds and the statistical properties of star formation that we can measure on extragalactic scales."

"Thus the present observations not only help resolve an outstanding question in Galactic star formation, but also help bring extragalactic and Galactic measurements of star formation into harmony," he added.