ALMA Captures the Birthplace of a Magnetized Protostellar Jet for the First Time

In the universe, stars and planets don’t form suddenly. Their formation resembles a lengthy construction process. Near a young star, there is often a surrounding disk of gas and dust called an accretion disk. Material in this disk keeps rotating, gathering together, and eventually falling onto the star, helping it grow over time. However, this process faces a major challenge: if the material in the accretion disk spins too quickly, it becomes hard for it to fall inward.

Astronomers have long believed that jets — streams of gas ejected at high speeds from near the star — can carry away the excess rotational energy, thereby easing the inward movement of material. However, the launching points of these jets are extremely close to the star, only tens of times closer than Earth is to the Sun, and previous observations have not been sufficient to resolve their details or clearly determine their origins.

An international research team led by Chin-Fei Lee at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe an extremely young protostar system called HH 211. This protostar is only about 35,000 years old, has just 6% of the Sun’s mass, and is located approximately 1,000 light-years away. It features a bright bipolar jet, and notably, this jet is one of the few known examples where a magnetic field has been detected, providing a rare opportunity to test models of magnetic-field–driven ejection.

The observations reveal that the jet moves at over 100 kilometers per second but rotates very slowly, with a specific angular momentum of only 4 au·km/s. Using conservation of angular momentum and energy, the team determined that the jet originates from the innermost edge of the accretion disk, just 0.02 astronomical units from the star — in excellent agreement with the theoretical X-wind model. This model explains how a magnetic field can act like a slingshot to propel gas outward, and it predicts a magnetic field strength consistent with previous measurements.

This discovery marks the first time the launch point of a magnetized jet has been identified with such high precision, directly confirming that jets are truly the “plumbers” of star formation—removing the last bits of angular momentum from the accretion disk so material can fall smoothly onto the star. In the future, these observations will not only help solve the mystery of how stars form but also enhance our understanding of the early stages of planet formation, since planets develop within these same disks.

Additional Information

This research was presented in a paper, “A magnetized protostellar jet launched from the innermost disk at the truncation radius,” by Lee et al., which appeared in Scientific Reports.

This release is adapted from the original Science Highlight issued by the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taiwan.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (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 (NSTC) in Taiwan, 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 ALMA's construction, commissioning, and operation.

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