Formation of the Hub-Filament System G33.92+0.11: Local Interplay
  between Gravity, Velocity, and Magnetic Field
  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 proto-clusters. Understanding the origin of HFSs is
crucial to reveal how star formation proceeds from clouds to cores. Here, we
report JCMT POL-2 850 $\mu$m polarization and IRAM 30-m C$^{18}$O (2-1) line
observations toward the massive HFS G33.92+0.11. The 850 $\mu$m 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.
