2025A&A...696A.163L

Aims. Fragmentation is a multiscale process forming structures with sizes that vary by several orders of magnitude. However, multiscale investigations of the magnetic field characterizing its properties across the physical scales relevant to the fragmentation process (filaments and clouds, clumps, and cores) are elusive. In this work, we present a multiscale study of the magnetic field using polarization continuum observations with various resolutions.
Methods. We made use of data from the JCMT and the SMA at 850 μm and 1.3 millimeter (mm) wavelengths to study the filamentary infrared dark cloud SDC18.624-0.070. Our observations cover filament (∼ 10 pc), filament-embedded clump (∼ 1 pc), isolated clump (∼ 0.1 pc), and clump-embedded core (∼ 0.01) scales, which are key to investigating the impact of the magnetic field on fragmentation.
Results. We found a magnetic field that is predominantly perpendicular to the major axes of all structures (filament, clumps, and cores). While its circular mean orientations are preserved within about 20°, a systematically increasing field dispersion toward smaller scales indicates the growing impact of gravity. Velocity gradients traced by N₂H⁺, with a resolution similar to that of the polarization observations, also tend to be perpendicular to the filament's major axis. All these features suggest that the magnetic field constrains the direction of accretion and initial contraction, as predicted by strong-field models.
Conclusions. We argue that the observed magnetic field and velocity gradient can result from a combination of converging flows, based on a detected SiO component along the filament, and rotation, based on the measured N₂H⁺ specific angular momentum profile. A multiscale energy analysis of gravity, magnetic field, and turbulence quantifying their relative importance shows that SDC18-S, despite displaying less fragmentation, has a larger field strength than SDC18-N, which harbors more fragments. A faster (SDC18-N) and slower evolution (SDC18-S) to a gravity-dominated regime has been found to explain the different fragmentation at clump-embedded core scale, with the stronger magnetic field in SDC18-S suppressing fragmentation to a greater extent.