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

Abstract 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 13 CO( 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">∣</mml:mo> <mml:mo stretchy="false">∣</mml:mo> </mml:mrow> </mml:msub> </mml:math> &gt; 10 −3 M ⊙ yr −1 ) to their hubs (i.e., Hub-1 and Hub-2, respectively). Hub-1 is comparatively massive, having higher <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">∣</mml:mo> <mml:mo stretchy="false">∣</mml:mo> </mml:mrow> </mml:msub> </mml:math> than Hub-2, allowing to derive a relationship <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">∣</mml:mo> <mml:mo stretchy="false">∣</mml:mo> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">hub</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>β</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">∣</mml:mo> <mml:mo stretchy="false">∣</mml:mo> </mml:mrow> </mml:msub> </mml:math> , the nondetection of radio continuum emission toward Hub-2 suggests it is in the relatively early stages of MSF. Analysis of 13 CO( 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.