Hierarchical fragmentation in hub-filament-system I18308 observed as part of the INFANT survey

Context. There is increasing evidence of a physical link between high-mass star formation and hub-filament systems (HFSs). However, a lack of multi-scale observations of HFS clouds hinders our understanding of the detailed and scale-dependent cloud fragmentation and associated dynamical high-mass star formation. Aims. This study aims to understand the multi-scale scenario of cloud fragmentation and associated high-mass star formation in an HFS cloud. Methods. As part of the ALMA-INFANT survey, we used 1.3 mm mosaic observations of the high-mass star-forming HFS cloud I18308 at a spatial resolution of ~3000 AU, which provided multiscale information on the HFS. We analyzed the filament and hub fragmentation properties (e.g., core separation and mass). Results. The I18308 cloud exhibits a well-defined HFS morphology in ALMA 1.3 mm continuum with two filaments (F1 and F2) converging toward the central hub. Eighteen compact cores are identified: nine in the hub, six in F1, and three in F2. Most cores are gravitationally bound and have high-mass surface densities of >1 g cm -2 , indicating their potential for high-mass star formation, especially in the hub, which already hosts an embedded UCH II region. The scale-dependent fragmentation is characterized by a cylindrical mode for F1 and F2, and a nearly-spherical Jeans-like mode for the central clumpy hub. This could be attributed to the (an)isotropic evolution of larger scale density structures into smaller scale ones. Additionally, the scale-dependent fragmentation mechanisms are identified as turbulence-driven within the filaments and gravity-driven inside the central hub. No candidate high-mass prestellar cores (>30 M ⊙ ) are observed across the whole cloud. In the hub, protostellar cores have higher average mass, surface density, and temperature; and smaller radius than prestellar cores, which is consistent with continuous mass accumulation during evolution. Conclusions. The well-defined HFS morphology, the absence of high-mass prestellar cores, and the increasing core mass and surface density with evolutionary stage collectively suggest a multi-scale dynamical scenario of mass accumulation for high-mass star formation in I18308.