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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.

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 young star and the forming disk. We’ve long known that planets form in flattened, spinning disks of gas and dust around young stars. But exactly how the material falling from the massive outer cloud "hits the brakes" and organizes into a stable, spinning disk has been a major puzzle.
In this work, we, for the first time, investigate the envelope–disk transition zone (ENDTRANZ), which is the region where collapsing molecular cloud material transitions into a rotationally supported protoplanetary disk. Using global MHD disk simulations alongside ALMA Large Program eDisk (Embedded Disks in Planet Formation) observations, we show that this transition is not abrupt, but instead occurs through a radially extended zone marked by a “jump” in specific angular momentum. This jump reflects a shift from infall-dominated motion to Keplerian rotation, and serves as a kinematical tracer of angular momentum redistribution during star–disk formation.
We, for the first time, identify a jump in the specific angular momentum profile at the envelope-disk transition in the Class 0/I protostar L1527 IRS using ALMA eDisk data, linking theory directly with observations. This novel kinematical tracer, in the form of a jump in the specific angular momentum profile, essentially manifests from the gradual transition in the rotational velocity. This jump serves as the observational evidence for the existence of ENDTRANZ in L1527 IRS. The results suggest that internal gravitational torques play a dominant role in shaping disk formation, offering new insights into how protostellar systems evolve and how initial conditions for planet formation are set.
This work contributes to a broader effort to understand how collapsing star-forming clouds ultimately become planetary systems.
Please refer to ASIAA's science highlight for a full read on this research.
This research was published in an article titled “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.