ALMA Observes Magnetic Braking during Star Formation for the First Time

Released: 3rd February, 2026, Academia Sinica, Institute of Astronomy & Astrophysics (ASIAA), Taiwan

During the process of star formation, an accretion disk forms around a protostar as gas from the surrounding envelope collapses inward. However, if the rotating gas spins too fast, it cannot be captured by the gravity of the protostar and will instead be flung outward. Observations show that the sizes of accretion disks around most low-mass protostars are typically only about 10 to 100 au. This implies that, during the transition between the envelope and the accretion disk, there must be a mechanism that efficiently removes excess angular momentum. Astronomers have long suggested that magnetic fields, through a process known as magnetic braking, play a key role in regulating angular momentum, but direct observational evidence has been lacking.

Under the supervision of Dr. Chin-Fei Lee at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), Jyun-Heng Lin, then an undergraduate student at National Taiwan Normal University (NTNU), used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the young star-forming system HH 111 VLA 1 with C¹⁸O (J=2-1) molecular line. HH 111 VLA 1 is a well-studied protostellar system in Orion that hosts a sizable disk of about 160 au, deeply embedded within an infalling envelope. Previous studies have found a significant decrease in angular momentum within the envelope before the disk is formed.

Using observational data that cover a more complete range of spatial scales, the team was able, for the first time, to clearly detect a sudden drop in gas rotation velocity at the transition between the envelope and the Keplerian disk. The team found that at a radius of about 600 au from the protostar, the rotation velocity of the gas is only about 95% of the value expected from angular momentum conservation, forming a feature known as a “rotation dip.” This indicates that a mechanism is efficiently transporting angular momentum from the inner regions to the outer regions, with the most plausible explanation being magnetic braking caused by magnetic fields.

As the rotating gas spins rapidly and collapses inward, it drags surrounding magnetic field lines with it. This interaction generates a force from the magnetic field that acts like a rubber band pulling on the gas, slowing its rotation and allowing the gas to continue accumulating inward to form an accretion disk. These observational results are highly consistent with magnetohydrodynamic (MHD) simulations, allowing astronomers, for the first time, to directly witness how magnetic braking affects gas motions and removes angular momentum in a protostellar system.

This result not only helps us better understand how stars and planetary disks are formed, but also brings us closer to uncovering the key processes involved in the early formation of our Solar System. In the future, combining these observations with polarization measurements of both the envelope and the accretion disk will allow astronomers to directly map the structure of magnetic fields and further confirm the role of magnetic braking in star formation.

"Clear velocity changes at the envelope–disk transition have been difficult to observe. This discovery highlights the key role of magnetic fields in star formation, and we hope to identify similar signatures in more systems,” said Jyun-Heng Lin, currently a graduate student at National Tsing Hua University and the lead author of this study.

"Using higher-resolution ALMA observations, we have followed up on this system and resolved the region where this angular-momentum decrease occurs," commented Chin-Fei Lee, the deputy director of ASIAA.

The schematic diagram illustrates how magnetic field lines are dragged by the motion of the gas. In the transition region, gas rotation twists the magnetic field lines. These twisted field lines act like a stretched rubber band, exerting a magnetic torque that slows down the gas rotation and allows an accretion disk to form near the center of the protostellar system. Credit: Jyun-Heng Lin

This plot shows the distribution of distance from the protostar and the corresponding gas rotation velocity along the disk major axis, extending from the envelope toward the center. The x-axis represents the distance from the protostar, and the y-axis represents the gas rotation velocity. The blue solid curve shows the observed rotation velocity, the purple dashed curve shows the expected rotation velocity assuming conservation of angular momentum, and the magenta dash-dotted curve shows the Keplerian rotation velocity. Two vertical black dashed lines divide the structure into three regions: from outer to inner, the envelope, the transition region between the envelope and the disk, and the inner Keplerian disk. Credit: Lin et al.

More Information:

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organization for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council of Taiwan (NSTC), and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

This research was presented in a paper “The Rotation Dip in the Envelope–Disk Transition of HH 111: Evidence for Magnetic Braking” by Lin et al. appeared in the Astrophysical Journal Letters on December 8th, 2025.

Media Contact:

Dr. Chin-Fei Lee, Email: cflee@asiaa.sinica.edu.tw , Tel: +886 2 2366 5445