September 11, 2026

New simulations show that a white dwarf torn apart by an intermediate-mass black hole can detonate into a thermonuclear supernova. This explosion resembles those from double-white-dwarf mergers, but its brightness and spectra vary dramatically depending on viewing angle.

The study, recently published in Astronomy & Astrophysics, was led by CITA Postdoctoral Fellow and from the Institut d’Astrophysique de Paris, as well as co-authored by Taeho Ryu and Rüdiger Pakmor. The team combined high-resolution hydrodynamic simulations with detailed radiative-transfer modelling to predict, for the first time at this level of detail, what these rare events would actually look like from Earth.

A white dwarf’s close call

Intermediate-mass black holes — weighing in at somewhere between 100 and 100,000 times the mass of the Sun — are thought to lurk in the dense cores of some star clusters, but they are notoriously difficult to detect directly. One promising way to find them is to catch them in the act of tidally disrupting a passing star. Because white dwarfs are so dense, they can only be torn apart by black holes in this intermediate mass range; if a white dwarf strays too close to a much larger supermassive black hole, it disappears without a trace.

Using the moving-mesh hydrodynamics code AREPO, equipped with a 55-isotope nuclear reaction network, the team modeled the disruption of a compact carbon-oxygen white dwarf by a black hole roughly 500 times the Sun’s mass, varying how close the encounter was. This was followed by radiative-transfer calculations using the codes CMFGEN and LONG_POL to understand the observations of such events. Their key findings:

• A vast array of outcomes: The closest encounters compress the white dwarf so violently that runaway nuclear fusion ignites, converting anywhere from about 1% to as much as 82% of the star’s material into radioactive nickel-56 — the same isotope that powers ordinary Type Ia supernovae — depending on exactly how close the pass was. More distant encounters simply disrupt the star without triggering an explosion.
• A spectrally-diverse, lopsided explosion:  The disrupted debris has an onion-shell-layered structure of isotopes and fans out towards one direction, material spreading across an extremely wide range of velocities.
• It depends where you’re standing: Radiative-transfer modeling of the resulting light shows rise times and peak brightness comparable to normal Type Ia supernovae — but with an unusually strong dependence on viewing angle, meaning the same explosion could look very different to observers positioned in different directions. Models of spectra reveal a diverse range of potentially observable spectral lines depending on the extent of nuclear burning in the disrupted white dwarf. Months later, characteristic emission lines in the fading afterglow can appear shifted and distorted by thousands of kilometers per second, offering a potential telltale signature of these events.

Why it matters

Because white dwarfs cannot survive close encounters with far larger supermassive black holes, catching one of these disruption events in progress would be strong, direct evidence for the presence of an elusive intermediate-mass black hole — the missing link between stellar-mass and supermassive black holes. With next-generation surveys such as the Vera C. Rubin Observatory expected to discover far more tidal disruption events than ever before, the distinctive light curves, spectra, and viewing-angle dependence identified in this study could help astronomers pick out these rare white dwarf disruptions from the broader population of transients — and, in the process, help pin down where intermediate-mass black holes are hiding.

Publication details

Pavan Vynatheya, Luc Dessart, Taeho Ryu, and Rüdiger Pakmor, “Asymmetrical thermonuclear supernovae triggered by the tidal disruption of white dwarfs,” accepted for publication in Astronomy & Astrophysics, 2026. 

Media Contact:
Lyuba Encheva, Communications and Events Coordinator
Canadian Institute for Theoretical Astrophysics, University of Toronto
Email: communication@cita.utoronto.ca

September 9, 2026

Magnetohydrodynamic modelling shows that colliding and merging stars produce distinct “fingerprints,” offering a new way to trace the violent origins of unusual stars such as blue stragglers.

A study published recently in The Astrophysical Journal provides the most detailed comparison yet of what happens when two stars collide versus when two stars in a binary system merge together — two processes long suspected of producing similar-looking, rejuvenated stars, but which turn out to leave behind different physical signatures.

The research, led by CITA Postdoctoral Fellow along with Taeho Ryu, Chen Wang, Alison Sills, and Rüdiger Pakmor, used the moving-mesh magnetohydrodynamics code AREPO to simulate collisions and mergers of main-sequence stars between five and ten times the mass of the Sun. The team then compared the internal structure, rotation, chemical mixing, and magnetic fields of the resulting objects in detail.

Two roads, two different destinations

Both stellar collisions — sudden, high-speed encounters typically occurring in the crowded cores of dense star clusters — and binary mergers — the slower coalescence of two stars after unstable mass transfer — are known to produce more massive, rapidly rotating stars with hydrogen-enriched cores. These “rejuvenated” stars are prime candidates for explaining blue stragglers, a class of stars that appear anomalously bright and hot for their host clusters, as well as other exotic objects such as magnetic massive stars.

However, it has been difficult to tell these two formation pathways apart. The new simulations identify several distinguishing features:

• Chemical mixing: Merger products retain core hydrogen fractions up to 10% higher than collision products, reflecting differences in how thoroughly the two processes mix stellar material.
• Magnetic field structure: Turbulent mixing dramatically amplifies magnetic fields (by nine to twelve orders of magnitude) in both scenarios, but the character of the resulting fields differs: near-head-on (small-impact-parameter) collisions generate small-scale, tangled magnetic fields likely to fade over time, while mergers and grazing (large-impact-parameter) collisions generate large-scale, ordered fields that may persist far longer.
• Outflows: Only merger products in the study produced magnetically driven, bipolar outflows, with material streaming away at speeds up to 300 kilometers per second — a feature entirely absent from the collision products.

Why it matters

Distinguishing collision products from merger products has been a long-standing challenge for astronomers trying to interpret observations of blue stragglers, magnetic OB stars, and other objects thought to form through stellar interactions. Because collisions dominate in dense globular clusters while mergers are thought to be more common in sparser open clusters, being able to identify a star’s formation history from its magnetic and structural properties could sharpen models of how star clusters evolve and how exotic, highly magnetized stars, and potentially magnetars, come to be.

The authors note that the long-term evolutionary consequences of these differences, particularly whether the ordered magnetic fields in merger products survive over stellar lifetimes, warrant further study.

Publication details

Pavan Vynatheya et al 2026, “The Collision and Merger Products of Stars Do Not Look Alike: A Magnetohydrodynamics Comparison,” The Astrophysical Journal, Volume 999, Number 1. .

Media Contact:
Lyuba Encheva, Communications and Events Coordinator
Canadian Institute for Theoretical Astrophysics, University of Toronto
Email: communication@cita.utoronto.ca

June 1, 2026

New research performed on the at the University of Toronto reveals a fundamentally different mechanism by which exploding stars shape the turbulent structure of our galaxy. The study, authored by , postdoctoral fellow at the Canadian Institute for Theoretical Astrophysics (CITA), was accepted for publication in . It challenges long-standing assumptions about how turbulence is generated and sustained in the interstellar medium.

Turbulence—the chaotic, swirling motion of gas—is a fundamental process in astrophysics, influencing everything from star formation to the transport of energy and magnetic fields across galaxies. For decades, scientists have largely assumed that turbulence in the Milky Way follows a classical cascade: energy injected at large scales breaks down into progressively smaller eddies (vortices or “swirls”), consistent with the well-known Kolmogorov picture.

However, this new work demonstrates that supernova remnants—the expanding shells of gas left behind by stellar explosions—can instead inject turbulence at small scales that, then, propagates upward to larger scales through an inverse cascade. This finding suggests that the dominant source of galactic turbulence may not operate in the way previously thought. Instead of a top-down energy transference (dying out of energy), it’s a bottom-up build-up. His research shows that supernova explosions create microscopic, chaotic swirls first. Those tiny swirls then merge and push their energy upward, eventually creating the massive, large-scale turbulence that shapes our entire galaxy.

“Our results show that the structure of turbulence in the galaxy carries a direct imprint of the physical instabilities that occur in supernova remnants,” said Dr. Beattie. “Rather than simply decaying or cascading downward, turbulence generated at the interfaces within these remnants can grow and reorganize, ultimately influencing dynamics on galactic scales.”

The research identifies the contact discontinuity—the unstable boundary between hot, shocked ejecta and cooler surrounding gas—as a key site of turbulence generation. Instabilities and strong baroclinic effects at this interface drive the formation of vortical motions that seed turbulence at relatively small spatial scales. From there, nonlinear interactions transfer energy to larger scales, producing a turbulent spectrum that deviates significantly from classical expectations.

Crucially, these results were made possible by cutting-edge numerical simulations performed on the Trillium Supercomputer, one of Canada’s most advanced high-performance computing systems operated by SciNet at the University of Toronto. The scale and fidelity required to resolve both the small-scale instabilities and their large-scale consequences demand computational resources at the forefront of modern supercomputing.

This revised picture has broad implications. It provides a new framework for interpreting observations of interstellar turbulence, including velocity fluctuations and magnetic field structure, and may help explain why measured spectra in the Milky Way often differ from theoretical predictions. It also suggests that models of galaxy evolution and star formation may need to be re-evaluated to account for this alternative turbulent driving mechanism.

Beyond its implications for the Milky Way, the work points to a more general principle: that microscopic plasma processes and instabilities can leave lasting imprints on macroscopic astrophysical systems.

“This is a shift in perspective,” Dr. Beattie added. “If turbulence in galaxies is seeded at small scales by physical instabilities, then understanding those instabilities becomes essential for understanding the large-scale universe.”

The paper is available on arXiv: 

Beattie, J. 2026,��American Astronomical Society,��,��,��,��Supernovae Drive Large-scale, Incompressible Turbulence through Small-scale Instabilities. 

SCIENCE CONTACT: Dr. James Beattie

Media Contact:
Lyuba Encheva, Communications and Events Coordinator
Canadian Institute for Theoretical Astrophysics, University of Toronto
Email: communication@cita.utoronto.ca

April 15, 2026

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.

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 .

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SCIENCE CONTACT: Indrani Das (idas2@uwo.ca, idas@asiaa.sinica.edu.tw); Shantanu Basu (basu@uwo.ca,��basu@cita.utoronto.ca)

Media Contact:
Lyuba Encheva, Communications and Events Coordinator
Canadian Institute for Theoretical Astrophysics, University of Toronto
Email: communication@cita.utoronto.ca