How a Cosmic Cloud Becomes a Solar System: Scientists Discover the “EnDTranZ” Missing Link
A conceptual visualization of EnDTranZ, the transition zone at the envelope–disk boundary, which is shown as a red colored, belt-like annulus where the gas motion gradually transitions from the infalling envelope to the Keplerian rotation within the protoplanetary disk surrounding a young star. This is an AI-generated illustration based on a two-dimensional spatial map of the specific angular momentum in the equatorial plane, as obtained from Das’s numerical simulations. The specific angular momentum map offers an intuitive lens to ‘see’ EnDTranZ, making its dynamics more apparent than in the rotational velocity map. Image Credit: Indrani Das/ASIAA.
Shantanu Basu, Interim Director of the Canadian Institute for Theoretical Astrophysics, is part of an international team that has revealed, for the first time, how infalling gas from star-forming cores gradually transitions into planet-forming disks. The study, led by Indrani Das from Academia Sinica Institute of Astronomy and Astrophysics, combines numerical simulations with observations from the Atacama Large Millimeter/submillimeter Array (ALMA). Findings were published today in the Astrophysical Journal.
Protoplanetary disks form around young stars when dense molecular cloud cores collapse under their own gravity. An outer shroud of gas and dust, known as the envelope, surrounds and feeds both the star and the forming disk. While it is well understood that planets eventually form within these disks and follow Keplerian orbits (the elliptical trajectory of an object revolving around a much more massive central object due to gravity), the mechanism that transforms infalling gas from the envelope into Keplerian motion (a highly organized, stable rotation) within the disk has remained a mystery for decades.
Based on both theoretical and observational evidence, the study establishes the existence of a distinct transition zone in which infalling gas transforms into a rotating disk. Lead author Indrani Das named this transition region EnDTranZ (Envelope Disk Transition Zone). The findings also reveal that infalling gas motions change gradually rather than abruptly into Keplerian motions, thus contradicting earlier infall models based on classical test-particle dynamics.
“The existence of EnDTranZ naturally results from the redistribution of mass and angular momentum during the formation of disks around young stars. This process ultimately governs how infalling material from the envelope, which rotates more slowly than the Keplerian speed, spreads out to form the disk and gradually settles into ordered Keplerian rotation”, explained Das, emphasizing that the discovery of EnDTranZ is a major step forward in understanding how stars and planetary systems—including our own Solar System—form.
The young protostellar system L1527 IRS taken with NIRCam on the James Webb Space Telescope (left panel), and the observed gas motions in this system obtained by the ALMA Large Program eDisk (right panel). (a) The radial variation of the specific angular momentum and (b) rotational velocity are shown based on the blue- and red-shifted velocity components. A jump in the observed radial profile of specific angular momentum at the region highlighted in orange color is the evidence of ENDTRANZ where the gas motion transitions from the infalling-rotating envelope to the Keplerian disk. Image Credits: (left) NASA, ESA, CSA, STScI; (right) Indrani Das/ASIAA.
To determine the physics of EnDTranZ, the team first ran numerical simulations using FEOSAD, a code that models a star-disk system starting from the collapse of a starless cloud core. The simulations revealed a unique fingerprint: as the gas crosses the transition zone, there is a distinct, measurable “jump” in its specific angular momentum (a physical measurement of how fast the gas is spinning relative to its distance from the star). The team identified this jump as a novel “kinematic signature” and a reliable way to track and locate EnDTranZ in the universe.
“This ‘jump’ in the data is caused by a gradual change in how fast the gas is spinning,” said Basu, a co-author of the study. “Tracking this change in rotation gives us a powerful diagnostic tool to understand the physical forces driving the evolution of these planet-forming disks.”
The team also studied L1527 IRS, a young star located 450 light-years from Earth in the Taurus molecular cloud, which hosts a disk with a radius of approximately 70 astronomical units. Using high-resolution ALMA Large Program eDisk (Embedded Disks in Planet Formation) observations, the researchers identified a similar jump in the radial profile of specific angular momentum at the system’s envelope-disk transition. Spanning a radial width of about 16 astronomical units, this observed jump confirmed the existence of a transition zone.
“At first, I did not believe that the observational data of L1527 IRS showed evidence of EnDTranZ, but surprisingly, it was there! A careful inspection and comparison of the radial dependence of specific angular momentum between the observational data and the simulation helped identify the evidence of EnDTranZ in L1527 IRS,” said Ohashi, the principal investigator of the ALMA eDisk large program and another co-author of this study.
“Interestingly enough, model EnDTranZ exhibits significant local variations in kinematics around the disk circumference and, when combined with observations, can offer insights into the complex spiral structure of a protoplanetary disk,” commented Vorobyov, another co-author of the study.
This pioneering work establishes EnDTranZ as a new frontier in star and planet formation studies, opening the door to deeper exploration of its complex physics and to searching for its signatures in other young stellar systems. As the team notes, this is just the beginning.
Research team: The research team includes Dr. Indrani Das from Academia Sinica Institute of Astronomy and Astrophysics (Taiwan), Prof. Shantanu Basu from the Canadian Institute for Theoretical Astrophysics (Canada), Prof. Nagayoshi Ohashi from Academia Sinica Institute of Astronomy and Astrophysics (Taiwan), Dr. Eduard Vorobyov from the University of Innsbruck (Austria), and Dr. Yusuke Aso from the Korea Astronomy and Space Science Institute (Korea).
Original paper: “Modelling the Break in the Specific Angular Momentum within the Envelope-Disk Transition Zone” by Das, I. et al. in the Astrophysical Journal, with a DOI: 10.3847/1538-4357/ae4725.
Read more in ASIAA Science Highlights, ALMA Telescope News, ALMA-Chile and the University of Innsbruck Newsroom.
SCIENCE CONTACT: Indrani Das (idas2@uwo.ca, idas@asiaa.sinica.edu.tw); Shantanu Basu (basu@uwo.ca, basu@cita.utoronto.ca)
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Canadian Institute for Theoretical Astrophysics, University of Toronto
Email: communication@cita.utoronto.ca