G321.93-0.01: A Rare Site of Multiple Hub-filament Systems with Evidence of Collision and Merging of Filaments

Hub-filament systems (HFSs) are potential sites of massive star formation (MSF). To understand the role of filaments in MSF and the origin of HFSs, we conducted a multiscale and multiwavelength observational investigation of the molecular cloud G321.93-0.01. The ¹³CO(J = 2-1) data reveal multiple HFSs, namely, HFS-1, HFS-2, and a candidate HFS (C-HFS). HFS-1 and HFS-2 exhibit significant mass accretion rates ( M ̇ ∣ ∣ > 10⁻³M_⊙ yr⁻¹) to their hubs (i.e., Hub-1 and Hub-2, respectively). Hub-1 is comparatively massive, having higher M ̇ ∣ ∣ than Hub-2, allowing to derive a relationship M ̇ ∣ ∣ ∝ M hub β , with β ~ 1.28. Detection of three compact H ii regions within Hub-1 using MeerKAT 1.28 GHz radio continuum data and the presence of a clump (ATL-3), which meets Kauffmann and Pillai's criteria for MSF, confirm the massive star-forming activity in HFS-1. We find several low-mass Atacama Large Millimeter/submillimeter Array cores (1-9 M_⊙) inside ATL-3. The presence of a compact H ii region at the hub of C-HFS confirms that it is active in MSF. Therefore, HFS-1 and C-HFS are in relatively evolved stages of MSF, where massive stars have begun ionizing their surroundings. Conversely, despite a high M ̇ ∣ ∣ , the nondetection of radio continuum emission toward Hub-2 suggests it is in the relatively early stages of MSF. Analysis of ¹³CO(J = 2-1) data reveals that the formation of HFS-1 was likely triggered by the collision of a filamentary cloud about 1 Myr ago. In contrast, the relative velocities (≳1 km s⁻¹) among the filaments of HFS-2 and C-HFS indicate their formation through the merging of filaments.