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We investigate the presence of hub-filament systems in a large sample of 146 active proto-clusters, using H13CO+ J = 1-0 molecular line data obtained from the ATOMS survey. We find that filaments are ubiquitous in proto-clusters, and hub-filament systems are very common from dense core scales (∼0.1 pc) to clump/cloud scales (∼1–10 pc). The proportion of proto-clusters containing hub-filament systems decreases with increasing dust temperature (Td) and luminosity-to-mass ratios (L/M) of clumps, indicating that stellar feedback from H ii regions gradually destroys the hub-filament systems as proto-clusters evolve. Clear velocity gradients are seen along the longest filaments with a mean velocity gradient of 8.71 km s−1 pc−1 and a median velocity gradient of 5.54 km s−1 pc−1. We find that velocity gradients are small for filament lengths larger than ∼1 pc, probably hinting at the existence of inertial inflows, although we cannot determine whether the latter are driven by large-scale turbulence or large-scale gravitational contraction. In contrast, velocity gradients below ∼1 pc dramatically increase as filament lengths decrease, indicating that the gravity of the hubs or cores starts to dominate gas infall at small scales. We suggest that self-similar hub-filament systems and filamentary accretion at all scales may play a key role in high-mass star formation.

 

Context. We started a multi-scale analysis of star formation in G202.3+2.5, an intertwined filamentary sub-region of the Monoceros OB1 molecular complex, in order to provide observational constraints on current theories and models that attempt to explain star formation globally. In the first paper (Paper I), we examined the distributions of dense cores and protostars and found enhanced star formation activity in the junction region of the filaments. Aims. In this second paper, we aim to unveil the connections between the core and filament evolutions, and between the filament dynamics and the global evolution of the cloud. Methods. We characterise the gas dynamics and energy balance in different parts of G202.3+2.5 using infrared observations from the Herschel and WISE telescopes and molecular tracers observed with the IRAM 30-m and TRAO 14-m telescopes. The velocity field of the cloud is examined and velocity-coherent structures are identified, characterised, and put in perspective with the cloud environment. Results. Two main velocity components are revealed, well separated in radial velocities in the north and merged around the location of intense N 2 H + emission in the centre of G202.3+2.5 where Paper I found the peak of star formation activity. We show that the relative position of the two components along the sightline, and the velocity gradient of the N 2 H + emission imply that the components have been undergoing collision for ~10 5 yr, although it remains unclear whether the gas moves mainly along or across the filament axes. The dense gas where N 2 H + is detected is interpreted as the compressed region between the two filaments, which corresponds to a high mass inflow rate of ~1 × 10 −3 M ⊙ yr −1 and possibly leads to a significant increase in its star formation efficiency. We identify a protostellar source in the junction region that possibly powers two crossed intermittent outflows. We show that the H  II region around the nearby cluster NCG 2264 is still expanding and its role in the collision is examined. However, we cannot rule out the idea that the collision arises mostly from the global collapse of the cloud. Conclusions. The (sub-)filament-scale observables examined in this paper reveal a collision between G202.3+2.5 sub-structures and its probable role in feeding the cores in the junction region. To shed more light on this link between core and filament evolutions, one must characterise the cloud morphology, its fragmentation, and magnetic field, all at high resolution. We consider the role of the environment in this paper, but a larger-scale study of this region is now necessary to investigate the scenario of a global cloud collapse. 

ABSTRACT The ATOMS, standing for ALMA Three-millimeter Observations of Massive Star-forming regions, survey has observed 146 active star-forming regions with ALMA band 3, aiming to systematically investigate the spatial distribution of various dense gas tracers in a large sample of Galactic massive clumps, to study the roles of stellar feedback in star formation, and to characterize filamentary structures inside massive clumps. In this work, the observations, data analysis, and example science of the ATOMS survey are presented, using a case study for the G9.62+0.19 complex. Toward this source, some transitions, commonly assumed to trace dense gas, including CS J = 2−1, HCO+J = 1−0, and HCN J = 1−0, are found to show extended gas emission in low-density regions within the clump; less than 25 per cent of their emission is from dense cores. SO, CH3OH, H13CN, and HC3N show similar morphologies in their spatial distributions and reveal well the dense cores. Widespread narrow SiO emission is present (over ∼1 pc), which may be caused by slow shocks from large–scale colliding flows or H ii regions. Stellar feedback from an expanding H ii region has greatly reshaped the natal clump, significantly changed the spatial distribution of gas, and may also account for the sequential high-mass star formation in the G9.62+0.19 complex. The ATOMS survey data can be jointly analysed with other survey data, e.g. MALT90, Orion B, EMPIRE, ALMA_IMF, and ALMAGAL, to deepen our understandings of ‘dense gas’ star formation scaling relations and massive protocluster formation.