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Peer-reviewedCC BY 4.0Astronomy & Astrophysics

Magnetic field morphology and strength in the high-mass star-forming clump G31.41+0.31

  • Greta H. L. Siu1,2,3
  • Hauyu Baobab Liu4,5
  • Yuxin Lin6
  • Joaquin Zamponi6
  • Maria José Maureira6
  • Shih-Ping Lai1
  • Junhao Liu7
7 affiliations
  1. 1Institute of Astronomy, National Tsing Hua University
  2. 2Department of Physics, The Chinese University of Hong Kong
  3. 3Gerald Choa Neuroscience Institute, School of Biomedical Sciences, The Chinese University of Hong Kong
  4. 4Department of Physics, National Sun Yat-Sen University
  5. 5Center of Astronomy and Gravitation, National Taiwan Normal University
  6. 6Max-Planck-Institut für Extraterrestrische Physik
  7. 7National Astronomical Observatory of Japan
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Abstract

Context. The ∼0.1 pc hot molecular core (HMC) in the massive molecular clump G31.41+0.31 (G31) is one of the clearest examples of an hourglass-shaped magnetic field (B field) in a high-mass star-forming region. However, the B field in the ∼0.1–0.4 pc parent clump remains relatively unexplored at high angular resolutions.

Aims. We aim to investigate the multi-scale evolution of the B field in G31, including its morphology, strength, and dynamical importance in G31.

Methods. We performed the ∼2″ (∼0.04 pc) angular-resolution JVLA Q-band (∼7 mm) dust polarization observations toward G31, which represents one of the earliest dust polarization observations on high-mass star-forming regions at this wavelength. We jointly analyzed the Q-band observation with the previous higher-resolution ALMA 1.3 mm dust polarization observations. A simple radiative transfer model was used to determine the polarization mechanism observed. We used 3D full polarization radiative transfer modeling and synthetic observations to constrain the B-field morphology threading the G31 clump and core. Angular dispersion analysis was used to estimate average B-field strengths. The energetics was then estimated from the clump to the core.

Results. The JVLA observations resolve linearly polarized intensity with rather uniform polarization position angles in the inner ∼0.4 pc region, which indicates a relatively strong and uniform B field on ≲0.4 pc scales. We find that our 7 mm observations and the previous ALMA observations can be coherently explained with a multi-scale B-field configuration: a large-scale uniform field encompassing a central hourglass shape. The average B-field strength is estimated to be ∼8–11 mG. We estimated that from ∼0.4 pc to ∼0.04 pc scales, the magnetic critical parameter, λ, increases from 1.3 to 2.3, and the Alfvén Mach number, M A , increases from 0.4 to 1.5. This likely indicates a transition from a trans-critical, sub-Alfvénic regime at the clump scale to a supercritical, super-Alfvénic regime at the core scale.

Conclusions. A concordant explanation for these results is that the initially uniform B field is dragged by the rapid self-gravitational collapse around the HMC. The gravitational contraction and/or the (proto)stellar feedback may enhance turbulence around the HMC.

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Cite

BibTeXRIS
BibTeX
@article{siu2026magnetic,
  title = {{Magnetic field morphology and strength in the high-mass star-forming clump G31.41+0.31}},
  author = {Greta H. L. Siu and Hauyu Baobab Liu and Yuxin Lin and Joaquin Zamponi and Maria José Maureira and Shih-Ping Lai and Junhao Liu},
  journal = {Astronomy \& Astrophysics},
  year = {2026},
  month = nov,
  doi = {10.1051/0004-6361/202556484},
  url = {https://www.aanda.org/10.1051/0004-6361/202556484},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Magnetic field morphology and strength in the high-mass star-forming clump G31.41+0.31
AU  - Greta H. L. Siu
AU  - Hauyu Baobab Liu
AU  - Yuxin Lin
AU  - Joaquin Zamponi
AU  - Maria José Maureira
AU  - Shih-Ping Lai
AU  - Junhao Liu
PY  - 2026
DA  - 2026/11/04
JO  - Astronomy & Astrophysics
DO  - 10.1051/0004-6361/202556484
UR  - https://www.aanda.org/10.1051/0004-6361/202556484
AB  - Context. The ∼0.1 pc hot molecular core (HMC) in the massive molecular clump G31.41+0.31 (G31) is one of the clearest examples of an hourglass-shaped magnetic field (B field) in a high-mass star-forming region. However, the B field in the ∼0.1–0.4 pc parent clump remains relatively unexplored at high angular resolutions.  Aims. We aim to investigate the multi-scale evolution of the B field in G31, including its morphology, strength, and dynamical importance in G31.  Methods. We performed the ∼2″ (∼0.04 pc) angular-resolution JVLA Q-band (∼7 mm) dust polarization observations toward G31, which represents one of the earliest dust polarization observations on high-mass star-forming regions at this wavelength. We jointly analyzed the Q-band observation with the previous higher-resolution ALMA 1.3 mm dust polarization observations. A simple radiative transfer model was used to determine the polarization mechanism observed. We used 3D full polarization radiative transfer modeling and synthetic observations to constrain the B-field morphology threading the G31 clump and core. Angular dispersion analysis was used to estimate average B-field strengths. The energetics was then estimated from the clump to the core.  Results. The JVLA observations resolve linearly polarized intensity with rather uniform polarization position angles in the inner ∼0.4 pc region, which indicates a relatively strong and uniform B field on ≲0.4 pc scales. We find that our 7 mm observations and the previous ALMA observations can be coherently explained with a multi-scale B-field configuration: a large-scale uniform field encompassing a central hourglass shape. The average B-field strength is estimated to be ∼8–11 mG. We estimated that from ∼0.4 pc to ∼0.04 pc scales, the magnetic critical parameter, λ, increases from 1.3 to 2.3, and the Alfvén Mach number, M A , increases from 0.4 to 1.5. This likely indicates a transition from a trans-critical, sub-Alfvénic regime at the clump scale to a supercritical, super-Alfvénic regime at the core scale.  Conclusions. A concordant explanation for these results is that the initially uniform B field is dragged by the rapid self-gravitational collapse around the HMC. The gravitational contraction and/or the (proto)stellar feedback may enhance turbulence around the HMC.
SN  - 0004-6361
SN  - 1432-0746
ER  -