Letter to the Editor

A multiscale view of the magnetic field morphology in the hot molecular core G31.41+0.31

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
² Institute of Liberal Arts and Sciences Tokushima University, Minami Jousanajima-machi 1-1, Tokushima 770-8502, Japan
³ Institut de Ciències de l’Espai (ICE-CSIC), Campus UAB, Can Magrans S/N, E-08193 Cerdanyola del Vallès, Catalonia, Spain
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), Esteve Terradas 1, PMT-UPC, E-08860 Castelldefels, Catalonia, Spain
⁵ The Institute for Advanced Study, Kyushu University, Japan
⁶ Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
⁷ Division of Science, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy

^⋆ Corresponding author: chi.law@inaf.it

Received: 18 December 2024
Accepted: 3 April 2025

Abstract

Multiscale studies of the morphology and strength of the magnetic field are crucial to properly unveil its role and relative importance in high-mass star and cluster formation. G31.41+0.31 (G31) is a hub-filament system that hosts a high-mass protocluster embedded in a hot molecular core (HMC). G31 is one of the few sources showing a clear hourglass morphology of the magnetic field on scales between 1000 au and a few 100 au in previous interferometric observations. This strongly suggests a field-regulated collapse. To complete the study of the magnetic field properties in this high-mass star-forming region, we carried out observations with the James Clerk Maxwell Telescope 850 μm of the polarized dust emission. These observations had a spatial resolution of ∼0.2 pc at 3.75 kpc. The aim was to study the magnetic field in the whole cloud and to compare the magnetic field orientation toward the HMC from ∼50 000 au to ∼260 au scales. The large-scale (∼5 pc) orientation of the magnetic field toward the position of the HMC is consistent with that observed at the core (∼4000 au) and circumstellar (∼260 au) scales. The self-similarity of the magnetic field orientation at these different scales might arise from the brightest sources in the protocluster, whose collapse is dragging the magnetic field. These sources dominate the gravitational potential and the collapse in the HMC. The cloud-scale magnetic field strength of the G31 hub-filament system, which we estimated using the Davis-Chandrasekhar-Fermi method, is in the range 0.04–0.09 mG. The magnetic field orientation in the star-forming region shows a bimodal distribution, and it changes from an NW–SE direction in the north to an E–W direction in the south. The change in the orientation occurs in the close vicinity of the HMC. This favors a scenario of a cloud-cloud collision for the formation of this star-forming region. The different magnetic field orientations would be the remnant of the pristine orientations of the colliding clouds in this scenario.