Context. The formation of high-mass stars remains unknown in many aspects. Two families of models compete to explain the formation of high-mass stars. On the one hand, quasi-static models predict the existence of high-mass pre-stellar cores sustained by a high degree of turbulence. On the other hand competitive accretion models predict that high-mass proto-stellar cores evolve from low/intermediate mass proto-stellar cores in dynamic environments. Aims. The aim of the present work is to bring observational constraints at the scale of high-mass cores (∼0.03 pc). Methods. We targeted with ALMA and MOPRA a sample of 9 starless massive dense cores (MDCs) discovered in a recent Herschel/HOBYS study. Their mass and size (∼110 M and r=0.1 pc, respectively) are similar to the initial conditions used in the quasi-static family of models explaining for the formation of high-mass stars. We present ALMA 1.4mm continuum observations that resolve the Jeans length (λ Jeans ∼0.03 pc) and that are sensitive to the Jeans mass (M Jeans ∼0.65 M ) in the 9 starless MDCs, together with ALMA-12 CO(2-1) emission line observations. We also present HCO + (1-0), H 13 CO + (1-0) and N 2 H + (1-0) molecular lines from the MOPRA telescope for 8 of the 9 MDCs. Results. The 9 starless MDCs have the mass reservoir to form high-mass stars according to the criteria by Baldeschi et al. (2017). Three of the starless MDCs are subvirialized with α vir ∼0.35, and 4 MDCs show sign of collapse from their molecular emission lines. ALMA observations show very little fragmentation within the MDCs. Only two of the starless MDCs host compact continuum sources, whose fluxes correspond to < 3 M fragments. Therefore the mass reservoir of the MDCs has not yet been accreted onto compact objects, and most of the emission is filtered out by the interferometer. Conclusions. These observations do not support the quasi-static models for high-mass star formation since no high-mass pre-stellar core is found in NGC6334. The competitive accretion models, on the other hand, predict a level of fragmentation much higher than what we observe.

Context. Hub-filament systems are suggested to be the birth cradles of high-mass stars and clusters. Aims. We investigate the gas kinematics of hub-filament structures in the G333 giant molecular cloud complex using 13CO (3–2) observed with the APEX/LAsMA heterodyne camera. Methods. We applied the FILFINDER algorithm to the integrated intensity maps of the 13CO J = 3–2 line to identify filaments in the G333 complex, and we extracted the velocity and intensity along the filament skeleton from moment maps. Clear velocity and density fluctuations are seen along the filaments, allowing us to fit velocity gradients around the intensity peaks. Results. The velocity gradients we fit to the LAsMA and ALMA data agree with each other over the scales covered by ALMA observations in the ATOMS survey (<5 pc). Changes in velocity gradient with scale indicate a funnel structure of the velocity field in position-position-velocity (PPV) space. This is indicative of a smooth, continuously increasing velocity gradient from large to small scales, and thus is consistent with gravitational acceleration. The typical velocity gradient corresponding to a 1 pc scale is ~1.6 km s−1 pc−1. Assuming freefall, we estimate a kinematic mass within 1 pc of ~1190 M⊙, which is consistent with typical masses of clumps in the ATLASGAL survey of massive clumps in the inner Galaxy. We find direct evidence for gravitational acceleration from a comparison of the observed accelerations to those predicted by freefall onto dense hubs with masses from millimeter continuum observations. On large scales, we find that the inflow may be driven by the larger-scale structure, consistent with the hierarchical structure in the molecular cloud and gas inflow from large to small scales. The hub-filament structures at different scales may be organized into a hierarchical system extending up to the largest scales probed through the coupling of gravitational centers at different scales. Conclusions. We argue that the funnel structure in PPV space can be an effective probe for the gravitational collapse motions in molecular clouds. The large-scale gas inflow is driven by gravity, implying that the molecular clouds in the G333 complex may be in a state of global gravitational collapse.

We report Atacama Large Millimeter Array observations of 3 mm dust continuum emission and line emission, in HCO + , H 13 CO + , N 2 H + and CH 3 CN, towards two massive and dense clumps (MDCs) in early but distinct evolutionary phases (prestellar and protostellar), made with the goal of investigating their fragmentation characteristics at angular scales of ∼1 . Towards the prestellar clump we detected ten compact structures (cores), with radius from 1200 to 4500 AU and masses from 1.6 to 20 M . Half of these cores exhibit inverse P Cygni profiles in HCO + and are subvirialized indicating that they are undergoing collapse. Towards the protostellar clump we detected a massive (119 M ) central core, with a strong mass infall rate, and nine less massive cores, with masses from 1.7 to 27 M and radius from 1000 to 4300 AU. CH 3 CN rotational tem-Corresponding author: S., Neupane

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Context. Filamentary structures in the interstellar medium are closely related to star formation. It is possible that the dense gas mass fraction (DGMF) or clump formation efficiency in large-scale filaments determine whether or not they end up hosting star formation activity. Aims. We aim to automatically identify large-scale filaments, as well as characterizing them, investigating their association with Galactic structures, and studying their DGMFs. Methods. We used a modified minimum spanning tree (MST) algorithm to chain parsec-scale 13CO clumps previously extracted from the Structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium (SEDIGISM) survey. The MST connects nodes in a graph such that the sum of edge lengths is at a minimum. A modified MST also ensures the velocity coherence between nodes, so that the identified filaments are coherent in position-position-velocity (PPV) space. Results. We generated a catalog of 88 large-scale (>10 pc) filaments in the inner Galactic plane (with −60° < l < 18° and |b| < 0.5°). These SEDIGISM filaments are larger and less dense than MST filaments previously identified from the Bolocam Galactic Plane Survey (BGPS) and the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). We find that eight of the filaments run along spiral arms and can be regarded as the “bones” of the Milky Way. We also found three bones associated with the Local spur in PPV space. By compiling 168 large-scale filaments with available DGMF across the Galaxy, namely, an order of magnitude more than previously investigated, we find that DGMFs are not correlated with Galactic location. We also find that bones have higher DGMFs than other filaments.