Why is forming massive stars (O- and B-type) so difficult to understand?

Massive stars are much rarer and form much faster than Sun-like stars. Their intense radiation pressure is so strong that it can push away the very gas they need to grow, making it a major theoretical challenge to explain how they accumulate so much mass.

What is a 'molecular filament' in a stellar nursery?

A filament is a long, dense thread of interstellar gas and dust that can stretch for tens of light-years. These structures are now understood to be the primary sites of star formation, acting like cosmic 'conveyor belts' that channel material into dense star-forming cores.

What is the difference between 'competitive accretion' and 'turbulent core' models?

'Turbulent Core' models propose that a single, massive gas core collapses monolithically to form a massive star or binary. 'Competitive Accretion' models propose that many smaller protostars form in a cluster and compete for gas from a shared reservoir, with a lucky few growing into massive stars.

What is the significance of the DR21 Ridge for this research?

The DR21 Ridge is a nearby, extremely active, and massive filamentary hub. Its favorable orientation and rich population of young, massive protostars make it one of the best astrophysical laboratories in the galaxy for studying the complex process of high-mass star formation in detail.

How Massive Stars Form: A Multi-Scale Analysis of the DR21 Ridge

The formation of massive stars ($M > 8 M_\odot$) is a cornerstone of astrophysics, as these stars profoundly influence their galactic environment through powerful winds, intense radiation, and supernova explosions. However, the precise mechanisms by which they accumulate their mass remain highly debated. This publication presents a comprehensive, multi-scale analysis of the DR21 Ridge, an archetypal high-mass star-forming region, leveraging data from multiple observatories. Our aim is to connect the large-scale filamentary structure to the small-scale accretion dynamics of individual protostellar cores, providing a unified picture of this complex process.

Observations: A Multi-Wavelength, Multi-Scale Approach

To construct a complete picture of the DR21 Ridge, we combined observations that trace the region at three distinct physical scales.

Large-Scale Filament Structure via Far-Infrared Imaging

We utilized archival data from the Herschel Space Observatory's SPIRE instrument. These far-infrared maps trace the thermal emission from cold dust, allowing us to map the overall morphology of the DR21 main filament, which extends over 10 parsecs. This large-scale view is critical for understanding the global structure and mass reservoir of the entire star-forming complex.
Intermediate-Scale Gas Kinematics with Single-Dish Spectroscopy

Using the IRAM 30m telescope, we mapped the velocity field of the dense gas along the filament using molecular line tracers like N₂H⁺. This provided crucial kinematic information, revealing large-scale gas flows and velocity gradients that trace the gravitational potential of the filament, showing how material is being channeled towards the densest star-forming hubs.
Small-Scale Core Fragmentation with Millimeter Interferometry

Finally, we used the Atacama Large Millimeter/submillimeter Array (ALMA) to achieve sub-arcsecond resolution observations of the densest hub within the ridge. These data resolve the filament into a chain of individual, compact protostellar cores and allow us to study the gas motion and accretion signatures at the scale of individual forming stars.

Analysis I: The Hierarchical Fragmentation of the DR21 Main Filament

The Herschel data reveal that the DR21 Ridge is not a simple, uniform cylinder. Instead, it exhibits a hierarchical structure. The main filament contains several gravitationally bound, high-density clumps, or "hubs," with the most prominent being the central DR21(Main) hub. Analysis of the filament's mass per unit length shows that it is highly supercritical, far exceeding the threshold for gravitational stability. This indicates that the entire filament is undergoing global, hierarchical collapse, fragmenting into the dense hubs where star clusters will form.

Analysis II: Infall and Accretion onto Protostellar Cores

Our kinematic data provide direct evidence that these fragmented cores are actively accreting material from their surroundings at multiple scales.

Evidence for Large-Scale Longitudinal Accretion Flows

The IRAM 30m data reveal coherent velocity gradients along the spine of the DR21 filament, consistent with gas flowing longitudinally along the filament and into the central hubs. This confirms that the hubs are not just collapsing in isolation; they are being actively fed by a "conveyor belt" of material from the larger filamentary structure.
Kinematic Signatures of Gravitational Collapse in Dense Cores

The high-resolution ALMA data allow us to probe the gas motion within individual cores. In several cores harboring massive protostars, we detect inverse P-Cygni profiles in the spectra of infall tracer molecules like HCO⁺. This spectral signature—a redshifted absorption feature against a blueshifted emission peak—is unambiguous evidence of cold gas falling towards a central heating source, providing direct confirmation of ongoing gravitational collapse at the core scale.
High-Mass Protostars and Early Outflow Feedback

Within the most massive cores, we identify several high-mass protostars that are already luminous enough to power HII regions. Crucially, our ALMA data resolve powerful, collimated bipolar outflows being launched from these sources. These outflows are a key indicator of active disk-mediated accretion and represent the first signs of stellar feedback that will eventually disrupt the parent core.

Discussion: Competitive Accretion vs. Turbulent Core Models

Our multi-scale observations provide a crucial test for the two leading theories of massive star formation.

The 'Conveyor Belt' Role of Filamentary Accretion

The evidence for continuous gas flow from the large-scale filament into the central, fragmenting hub strongly supports models where the mass reservoir for the final stars is gathered over a wide area. This argues against purely monolithic collapse of an isolated, massive core.
Evidence for a Hybrid Formation Scenario

While the large-scale flows are consistent with competitive accretion, the ALMA data also show that the individual fragments are themselves very massive and turbulent, as predicted by Turbulent Core models. Our findings suggest a hybrid scenario: large-scale filaments gather material and fragment into massive, turbulent cores, which then continue to accrete both from their local envelope and from the shared gas reservoir of the parent hub, potentially engaging in competitive accretion with their siblings.

Conclusion: A Unified, Dynamic Picture of Massive Star Formation

This multi-scale analysis of the DR21 Ridge provides a unified, dynamic picture of high-mass star formation. The process is fundamentally hierarchical, beginning with the gravitational collapse of a large-scale filament, which then fragments into dense, cluster-forming hubs. These hubs are fed by continuous gas flows from the filament, and the individual protostellar cores within them accrete material from their immediate surroundings and the shared hub reservoir. This work demonstrates that understanding massive star formation requires connecting the physics of accretion across scales, from the ten-parsec filament down to the hundred-AU protostellar disk.