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Published: 22nd June, 2026, Academia Sinica, Institute of Astronomy & Astrophysics (ASIAA), Taiwan
Banner Image: NASA/ESA
Magnetic fields are believed to play a key role in the formation of stars, but they are extremely difficult to observe directly. Astronomers therefore rely on polarized light measurements to estimate magnetic field structures and strengths. A recent study led by Seamus Clarke at National Cheng Kung University, in collaboration with Ya-Wen Tang and Patrick Koch at Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), shows that two widely used analysis methods may systematically underestimate variations in polarization angles, leading scientists to overestimate magnetic field strengths and their influence on star formation. The findings were published in the Astrophysical Journal.
New stars are born inside giant clouds of gas and dust known as molecular clouds. Astronomers have long known that magnetic fields inside these clouds play an important role in the star formation process. However, magnetic fields are extremely difficult to observe directly, so scientists must rely on indirect methods to study them.
One common technique is to observe “polarized light” emitted by tiny dust grains. By analyzing the direction of this polarized light, astronomers can infer the structure of magnetic fields inside molecular clouds.
One study exploring the reliability of these measurements was led by Seamus Clarke, who began the project while working at ASIAA together with Ya-Wen Tang and Patrick Koch, in collaboration with Dr. Gary Fuller and Mr. Dawei Xi from University of Manchester. Their results suggest that two widely used analysis methods may systematically underestimate variations in polarization angles, causing magnetic field strengths to appear stronger than they actually are.
The research team analyzed simulated polarization maps and tested two common techniques: the “unsharp-masking” method and the “structure function” method. They found that both methods consistently underestimate the dispersion of polarization angles.
Why is this important?
Astronomers use the variation in polarization angles to estimate magnetic field strength. If the angle variations are underestimated, the magnetic field strength will be overestimated. This could lead scientists to believe that magnetic fields have a stronger influence on star formation than they truly do.
To address this issue, the team developed a new “correction factor” method designed to produce measurements that are closer to the true magnetic field conditions.
The researchers tested this method using observations of the famous star-forming region Orion A from the James Clerk Maxwell Telescope (JCMT). Their results showed that the magnetic field in this region is actually fairly well aligned, requiring only a modest correction factor of about 1.6 to compensate for the underestimation.
Although a factor of 1.6 may seem small, without the correction the magnetic field energy would have been overestimated by roughly 2.5 times, potentially giving astronomers a misleading picture of how stars form within the cloud.
This study highlights that even long-established observational techniques can contain hidden biases. By improving these methods, astronomers can gain a more accurate understanding of the true role magnetic fields play in the universe and how stars are formed.
The left panel shows the 850 micron emission from the Orion A OMC-1 region (cyan contour) seen by the JCMT, with the black segments showing the polarisation segments. The middle and right panel show the technique developed to determine the correction factor for the Orion A OMC-1 region. The method uses the measurement of polarisation angle dispersion on multiple spatial scales covered by the data to approximate the multi-scale structure of the magnetic field, and so estimate a correction factor to bring the polarisation angle dispersion measurement into alignment with the true underlying value. The observational data from Orion A is shown as the black line, with the individual numerical polarisation maps used as blue transparent lines, and their mean shown in orange. Image credit: Clarke et al.
More Information:
This research presented in a paper "Systematic underestimation of polarisation angle dispersion and its consequences for magnetic field strength estimates in star-forming regions,” by Clarke et al. has appeared in the Astrophysical Journal on May 28th, 2026.
Media Contact:
Dr. Ya-Wen Tang Email: ywtang@asiaa.sinica.edu.tw Tel: +886-2-2366-5448
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