2025A&A...703A..74K

Context. Hub-filament systems (HFSs) span a broad range of star-forming gas densities and are widely recognized as the progenitors of open star clusters. They serve as ideal targets for investigating the physical properties of star-forming gas and observing how dense gas is removed during the assembly of star clusters.
Aims. In this study, we explore the characteristics of three cluster-forming HFSs - W3(OH), W3 Main, and S 106 - that represent evolutionary stages from early to evolved, with a particular focus on the structure of their magnetic fields (B-field) and filament line-mass distributions. The goal is to identify indicators of the evolution of the HFSs, in particular, their hubs, as star formation proceeds.
Methods. Our analysis combines observations of dense star-forming gas and young stellar populations. We present new JCMT/POL-2 observations of 850 µm dust polarized emission to probe the dense gas and magnetic field structures. Additionally, we utilized archival infrared and radio data from WIRCAM, WFCAM, Spitzer, Herschel, and the VLA to identify markers of star formation. We derived radial column density profiles centred in the hubs and used them to define distinct filament and hub regions. We analysed istograms of line mass (M_line), polarization intensity (PI), polarization fraction (PF), and the relative orientation between the magnetic field and the filaments.
Results. Each hub contains two adjacent nodes or peaks of star formation, with one peak consistently more evolved than the other. The radial intensity profiles for all three targets fit well with two distinct power laws, -s between 0.6-0.8 pc; these define the semimajor axes of the hub, which approximates an elliptical shape. The power-law indices for the hub regions (0.6-0.8 pc) are -2.1, -1.7, and -0.9, and for the filament regions (>0.8 pc) they are -2.9, -4.2, and -11.7, corresponding to W3(OH), W3 Main, and S 106, respectively. These power-law slopes indicate different dynamical behaviours (where ≤-2 suggests global collapse), which is important to understand HFSs evolution. The hubs contain the highest line masses across all targets. In the earliest stage W3(OH), the filament line-mass function (FLMF) smoothly includes both the hub and filament regions in a Salpeter-like slope. In the evolved S 106, the hub FLMF slope is -0.85 and the filament region FLMF slope is -1.4. The plane-of-sky (POS) magnetic field structures display two notable features: (a) at low densities, B-field lines are misaligned with filaments but gradually align with them as density increases towards the hub; (b) B-field lines trace the walls of bipolar cavities formed by massive outflows from stars in the hub. PF and PI contour maps show disc and bipolar outflow-like patterns centred on the most luminous sources. Additionally, we identify a foreground mini-spiral HFS in W3 Main, previously recognized as the coldest clump in the region.
Conclusions. As HFSs evolve, discernible changes can be found in the FLMF, PF, and B-field-Filament angles, especially inside the hub, which is also found to increase in size. Massive bipolar outflows and radiation bubbles significantly reshape POS magnetic fields, aligning them along cavity walls and shells, adding to the well-documented rearrangements around HII region cavities. We notice there is an intriguing similarity between hub sizes and young cluster radii. The presence of 'double-node' star formation within hubs - characterized by systematic evolutionary differences - appears to be a common feature of HFSs. We present evidence for their widespread occurrence in several well-studied, nearby star-forming clouds.