2020ApJ...905..158W

The formation of filaments in molecular clouds is an important process in star formation. Hub-filament systems (HFSs) are a transition stage connecting parsec-scale filaments and protoclusters. Understanding the origin of HFSs is crucial to reveal how star formation proceeds from clouds to cores. Here we report James Clerk Maxwell telescope POL-2 850 µm polarization and IRAM 30 m C¹⁸O (2-1) line observations toward the massive HFS G33.92+0.11. The 850 µm continuum map reveals four major filaments converging to the center of G33.92+0.11 with numerous short filaments connecting to the major filaments at local intensity peaks. We estimate the local orientations of filaments, magnetic field, gravity, and velocity gradients from observations, and we examine their correlations based on their local properties. In the high-density areas, our analysis shows that the filaments tend to align with the magnetic field and local gravity. In the low-density areas, we find that the local velocity gradients tend to be perpendicular to both the magnetic field and local gravity, although the filaments still tend to align with local gravity. A global virial analysis suggests that the gravitational energy overall dominates the magnetic and kinematic energy. Combining local and global aspects, we conclude that the formation of G33.92+0.11 is predominantly driven by gravity, dragging and aligning the major filaments and magnetic field on the way to the inner dense center. Traced by local velocity gradients in the outer diffuse areas, ambient gas might be accreted onto the major filaments directly or via the short filaments.