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

¹ School of Physics and Astronomy, Yunnan University, Kunming 650091, PR China
² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
³ State Key Laboratory of Radio Astronomy and Technology, A20 Datun Road, Chaoyang District, Beijing 100101, PR China
⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia, Michoacán 58089, Mexico
⁶ Department of Physics, National Sun Yat-Sen University, No. 70, Lien-Hai Road, Kaohsiung City 80424, Taiwan, ROC
⁷ Department of Astronomy, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
⁸ The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
¹⁰ Max Planck Institute for Astronomy, Konigstuhl 17, 69117 Heidelberg, Germany
¹¹ Department of Astronomy, Xiamen University, Zengcuo’an West Road, Xiamen 361005, PR China
¹² Center for Astrophysics, Harvard & Smithsonian, MS-42, 60 Garden Street, Cambridge, MA 02138, USA
¹³ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
¹⁴ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching bei München, Germany
¹⁵ School of Astronomy and Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, PR China
¹⁶ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, PR China
¹⁷ Department of Earth and Planetary Sciences, Institute of Science Tokyo, Meguro, Tokyo 152-8551, Japan
¹⁸ Joint Alma Observatory (JAO), Alonso de Córdova 3107, Vitacura, Santiago, Chile

^★★ Corresponding authors: hongliliu2012@gmail.com; xinglv.nju@gmail.com; ycheng.astro@gmail.com

Received: 19 March 2025
Accepted: 4 June 2025

Abstract

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.