Free Astronomy Magazine July-August 2026

P revious page: A conceptual visualization of ENDTRANZ, the transition zone at the envelope–disk boundary, which is shown as a red colored, belt-like annu- lus 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 the nu- merical simulations. The specific angular momentum map offers an intuitive lens to ‘see’ ENDTRANZ, making its dynamics more apparent than in the rotational velocity map. [Indrani Das/ASIAA] 41 JULY-AUGUST 2026 ASTRO PUBLISHING Young stars are surrounded by a vast shroud of gas and dust called an envelope. Gravity pulls this ma- terial inward, feeding both the growing star and the disk around it. But the infalling gas moves differ- ently than the disk — more slowly and chaotically — and the point at which one becomes the other had never been clearly observed. Earlier theoretical models assumed the switch was sharp, al- most instantaneous. The new study shows it isn’t. Using numerical simula- tions with the FEOSAD code, the team tracked how a collapsing cloud core evolves into a star- disk system — and found that the transi- tion unfolds gradually across a finite region, leaving a tell-tale signa- ture: a characteristic “jump” in the distribu- tion of specific angular momentum, a measure of how gas rotates as a function of its distance from the star. “The existence of END- TRANZ naturally results from the redistribution of mass and angular mo- mentum during the for- mation of disks around young stars. This process ultimately governs how infalling material from the envelope, which ro- tates more slowly than they found exactly the same angu- lar momentum signature that the simulations had predicted — span- ning a zone roughly 16 astronomi- cal units wide, or about 16 times the distance from Earth to the Sun. “This ENDTRANZ tracer essentially manifests from the gradual transi- tion in the rotational velocity, which offers a diagnostic framework for understanding the physical proc- esses at play that drive the disk evo- lution,” said Shantanu Basu, Interim Director of the Canadian Institute for Theoretical Astrophysics and co- author of the study. ALMA’s extraordinary resolution was essential to making this detec- tion possible, resolving the structure at the precise interface between the envelope and the disk — a regime that had previously been beyond reach. “A careful inspection and comparison of the radial dependence of specific angular momentum between the observational da- ta and the simulations helped identify the evidence of END- TRANZ in L1527 IRS,” said Nagayoshi Oha- shi, principal investi- gator of the ALMA eDisk Large Program and co-author of the study. The discovery estab- lishes ENDTRANZ as a fundamental feature of how stars and plan- etary systems assem- ble — and opens the door to searching for the same signature in other young systems across the galaxy. “In many ways, we be- lieve this is just the be- ginning!” Das said. T he 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 high- lighted in orange color is the evidence of ENDTRANZ where the gas motion transitions from the infalling-rotating envelope to the Kep- lerian disk. [(left) NASA, ESA, CSA, STScI; (right) Indrani Das/ASIAA] the Keplerian speed, spreads out to form the disk and gradually settles into ordered Keplerian rotation,” explained Das. To test whether ENDTRANZ exists in nature, the team turned to L1527 IRS, a young protostar about 450 light-years away in the Taurus mo- lecular cloud. Using data from the ALMA Large Program eDisk (Em- bedded Disks in Planet Formation), !