Planet-building disks around newborn stars

Astronomers crack a decades-old mystery, catching gas morphing into planet-building disks around newborn stars

by Academia Sinica Institute of Astronomy and Astrophysics

An international team led by Dr. Indrani Das of Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) has shown, for the first time, how infalling gas from star-forming cores gradually transitions into planet-forming disks. Their findings, combining numerical simulations with Atacama Large Millimeter/submillimeter Array (ALMA) observations, are 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 young star and the forming disk. While it is well understood that planets eventually form within these disks and follow Keplerian orbits, the mechanism that transforms rapid infalling gas motion from the envelope into ordered Keplerian motion within the disk has remained a mystery for decades.

Based on both theoretical and observational evidence, the recent study discovered that there exists a distinct transition zone at the envelope-disk interface of a young star-disk system, which Das named ENDTRANZ (Envelope Disk Transition Zone). The findings have established that infalling gas motions gradually transition into Keplerian motions across this transition zone. Crucially, this transition is far from abrupt and contradicts earlier infall models that are 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.

To determine the physics of ENDTRANZ, the team first ran the numerical simulations using FEOSAD, a code that models the star-disk system starting from the collapse of a starless cloud core. Their results showed that the transition from the infalling-rotating envelope to the spinning disk gradually unfolds through a "jump" across a finite thickness in the radial profile of specific angular momentum, which they identified as a novel signature of ENDTRANZ.

Specific angular momentum is defined as the total angular momentum per unit mass, describing how fast and how far out a mass parcel orbits regardless of its mass. Thus it serves as a powerful tool for understanding how material rotating at different rates reorganizes during the evolution from collapsing gas clouds to disks. This systematic reorganization is analogous to atmospheric convection, where circulation occurs in an organized way, with warm air rising and cool air descending while exchanging heat.

"This ENDTRANZ tracer, in the form of a jump in the specific angular momentum profile, essentially manifests from the gradual transition in the rotational velocity. This change in rotational behavior offers a diagnostic framework for understanding the physical processes at play that drive the disk evolution," said Shantanu Basu, a co-author of the study.

The team also studied L1527 IRS, a young star located about 450 light-years from Earth in the Taurus molecular cloud, which hosts a disk with a radius of approximately 70 astronomical units. Using the high-resolution ALMA Large Program eDisk (Embedded Disks in Planet Formation) observations, the researchers, for the first time, identified a similar jump in the radial profile of the specific angular momentum at the envelope-disk transition of L1527 IRS. 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 Nagayoshi 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 Eduard 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. In many ways, the team believes this is just the beginning.