2023ApJ...957...61H

We present a dynamics study toward the G326.611+0.811 (G326) hub-filament system (HFS) cloud using new APEX observations of both ¹³CO and C¹⁸O (J = 2-1). The G326 HFS cloud constitutes a central hub and at least four hub-composing filaments that are divided into a major branch of filaments (F1 and F2) and a side branch (F3-F5). The cloud holds ongoing high-mass star formation as characterized by three massive dense clumps (i.e., 370-1100M_⊙ and 0.14-0.16 g cm^–2 for C1-C3) with high clump-averaged mass infalling rates (>10^–3M_⊙ yr^–1) within the major filament branch, and the associated point sources bright at 70 μm, typical of young protostars. Along the five filaments, velocity gradients are found in both ¹³CO and C¹⁸O (J = 2-1) emission, suggesting that filament-aligned gravitational collapse toward the central hub (i.e., C2) is responsible for the high-mass star formation therein. Moreover, a periodic velocity oscillation along the major filament branch is revealed in both ¹³CO and C¹⁸O (J = 2-1) emission with a characteristic wavelength of ∼3.5 pc and an amplitude of ∼0.31-0.38 km s^–1. We suggest that this pattern of velocity oscillation in G326 could arise from clump-forming gas motion induced by gravitational instabilities. The prevalent velocity gradients, fragmentation of the major branch of filaments, and the ongoing collapse of the three massive dense clumps are indicative that G326 is an HFS undergoing global collapse.