2022A&A...667A..90M

em Context. Filamentary infrared dark clouds (IRDCs) are a useful class of interstellar clouds for studying the cloud fragmentation mechanisms on different spatial scales. Determination of the physical properties of the substructures in IRDCs can also provide useful constraints on the initial conditions and early stages of star formation, including those of high-mass stars.

em Aims. We aim to determine the physical characteristics of two filamentary IRDCs, G1.75-0.08 and G11.36+0.80, and their clumps. We also attempt to understand how the IRDCs are fragmented into clumps.

em Methods. We imaged the target IRDCs at 350 and 450 µm using the bolometer called Architectures de bolomètres pour des Télescopes à grand champ de vue dans le domaine sub-Millimétrique au Sol (ArTéMiS). These data were used in conjunction with our previous 870 µm observations with the Large APEX BOlometer CAmera (LABOCA) and archival Spitzer and Berschel data. The LABOCA clump positions in G11.36+0.80 were also observed in the N₂H⁺(1-0) transition with the Institut de Radioastronomie Millimétrique (IRAM) 30-metre telescope.

em Results. On the basis of their far-IR to submillimetre spectral energy distributions (SEDs), G1.75-0.08 was found to be composed of two cold (∼14.5 K), massive (several ∼10³M_⊙) clumps that are projectively separated by ∼3.7 pc. Both clumps are 70 µm dark, but they do not appear to be bounded by self-gravity. The G1.75-0.08 filament was found to be subcritical by a factor of ∼14 with respect to its critical line mass, but the result is subject to uncertain gas velocity dispersion. The IRDC G11.36+0.80 was found to be moderately (by a factor of ∼2) supercritical and composed of four clumps that are detected at all wavelengths observed with the ground-based bolometers. The SED-based dust temperatures of the clumps are ∼13-15 K, and their masses are in the range ∼232-633 M_⊙. All the clumps are gravitationally bound and they appear to be in somewhat different stages of evolution on the basis of their luminosity-to-mass ratio. The projected, average separation of the clumps is ∼1 pc. At least three clumps in our sample show hints of fragmentation into smaller objects in the ArTéMiS images. Conclusions. A configuration that is observed in G1.75-0.08, namely two clumps at the ends of the filament, could be the result of gravitational focussing acting along the cloud. The two clumps fulfil the mass-radius threshold for high-mass star formation, but if their single-dish-based high velocity dispersion is confirmed, their gravitational potential energy would be strongly overcome by the internal kinetic energy, and the clumps would have to be confined by external pressure to survive. Owing to the location of G1.75-0.08 near the Galactic centre (∼270 pc), environmental effects such as a high level of turbulence, tidal forces, and shearing motions could affect the cloud dynamics. The observed clump separation in G11.36+0.80 can be understood in terms of a sausage instability, which conforms to the findings in some other IRDC filaments. The G11.36+0.80 clumps do not lie above the mass-radius threshold where high-mass star formation is expected to be possible, and hence lower-mass star formation seems more likely. The substructure observed in one of the clumps in G11.36+0.80 suggests that the IRDC has fragmented in a hierarchical fashion with a scale-dependent physical mechanism. This conforms to the filamentary paradigm for Galactic star formation.