A recent study published in Monthly Notices of the Royal Astronomical Society finds that the gas motions within SDC335.579-0.292—a massive, star-forming infrared dark cloud in the Milky Way—cannot be fully explained by the conventional assumption that collapse simply accelerates toward the center. Instead, the cloud exhibits an inverted infall velocity profile.

The study was led by Dr. XIE Jinjin of the star formation and evolution group at the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS), in collaboration with the University of Manchester, Tsinghua University, Cardiff University, and other institutions.

Massive stars shape the interstellar medium through intense radiation, stellar winds, and eventual supernova explosions. They play a key role in galaxy evolution and also influence the formation and evolution of nearby low-mass stars. Because massive stars form rapidly, how they acquire enough material in such a short time remains a central question in star formation.

SDC335.579-0.292, hereafter SDC335, lies about 3.25 kiloparsecs from the Sun, spans roughly 2.4 parsecs, and has a total mass of about 5,500 solar masses, with young OB stars forming inside. Its central source, SDC335-MM1, has been identified as one of the most massive compact protostellar cores known in the Galaxy, making SDC335 a key target for studying how massive stars acquire material from their parent clouds.

In observations of infall, optically thick lines such as HCO⁺ show self-absorption features where the blue-shifted peak is stronger than the red-shifted peak, and the systemic velocity—traced by the optically thin isotopologue H¹³CO⁺—appears in the self-absorption dip. Such line profiles provide strong evidence of inward gas motion.

In this work, the team analyzed spatially resolved HCO⁺ and H¹³CO⁺ spectral maps from the MALT90 survey, obtained with the Mopra 22-meter telescope. They then combined the observations with the Hill5 semi-analytical model and radiative transfer models, including LIME, RADMC-3D, and RATRAN, to infer the underlying velocity field. This multimodel comparison improves the reliability of the inferred infall velocities and highlights the limitations of simplified models in complex massive star-forming regions.

The best-fitting models constrain the infall velocity toward the cloud center to about 0.6–1.6 km·s⁻¹, with a mass infall rate of a few 10⁻³ to 10⁻² solar masses per year. Most importantly, the infall velocity does not increase monotonically toward the center. It reaches a minimum at about 0.7 parsecs and then increases again both inward and outward. This inverted velocity structure suggests that massive stars do not grow through a single, smooth radial inflow, but rather through mass transport regulated by cloud structure, fragmentation, filamentary gas flows, and local dynamics.

The researchers also revisit the interpretation of molecular linewidths. Line broadening is often attributed to turbulence, but in SDC335, the widths of optically thin lines may be strongly affected—or even dominated—by unresolved ordered infall motions within the telescope beam. These findings provide new observational constraints on global collapse and mass delivery in massive star formation.

This work was supported by the National Natural Science Foundation of China, the National Key R&D Program of China, the International Partnership Program of CAS, the Natural Science Foundation of Xinjiang Uygur Autonomous Region, the Regional Collaborative Innovation Project of Xinjiang, the Tianshan Talent Training Program, and the Tianchi Talent Project of Xinjiang.