Cocoons of Dying Stars Could Explain Mysterious Fast Blue Optical Transients

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The cocoons of dying stars may explain the fast blue optical transients. In this animation, a cocoon surrounds the jet of a collapsing star. As this cocoon escapes from the star, it cools, releasing heat in the form of FBOT emission. Credit: Ore Gottlieb/Northwestern University

First model fully consistent with all observations of fast blue optical transients.

New simulations developed by[{” attribute=””>Northwestern University’s Ore Gottlieb and Sasha Tchekovskoy present a potential explanation for the origins of a mysterious phenomenon called fast blue optical transients, or FBOTs. The model shows a massive star collapsing, launching outflows of debris at rates near the speed of light. These outflows, or jets, collide into collapsing layers of the dying star to form a “cocoon” around the jet. The new model shows that as the jet pushes the cocoon outward — away from the core of the collapsing star — it cools, releasing heat as an observed FBOT emission.

Ever since they were discovered in 2018, fast blue optical transients (FBOTs) have completely surprised and utterly perplexed both observational and theoretical astrophysicists.

These mysterious objects, which are so hot that they glow blue, are the brightest known optical phenomenon in the universe. However, with only a few discovered thus far, the origins of FBOTs have remained elusive.

Now a Northwestern University team of astrophysics presents a bold new explanation for the origin of these curious anomalies. Using a new model, these scientists believe FBOTs could result from the actively cooling cocoons that surround jets launched by dying stars. It marks the first astrophysics model that is fully consistent with all FBOT observations to date.

The research was published on April 11, 2022, in the journal Monthly Notices of the Royal Astronomical Society.


Full simulation of a collapsing star and launching jets, which collide with stellar material to form a cocoon. As the cocoon cools, it releases heat in the form of FBOT emission. Credit: Ore Gottlieb/Northwestern University

When a massive star collapses, it can launch debris flows at speeds close to the speed of light. These outflows, or jets, collide with the collapsing layers of the dying star to form a “cocoon” around the jet. The new model shows that as the jet pushes the cocoon outward – away from the collapsing star’s core – it cools, releasing heat as the observed FBOT emission.

“A jet starts deep inside a star and then pushes its way out,” said Northwestern’s Ore Gottlieb, who led the study. “As the jet travels through the star, it forms an extended structure, known as a cocoon. The cocoon envelops the jet, and it continues to do so even after the jet has escaped the star, which cocoon escapes with the jet. When we calculated the energy amount of the cocoon, it turned out to be as powerful as an FBOT.”

Dying Star Throw

Jet piercing through the stellar layers of a dying star. Credit: Ore Gottlieb/Northwestern University

Gottlieb is a Rothschild Fellow at Northwestern’s Center for Interdisciplinary Astrophysics Exploration and Research (CIERA). He co-authored the paper with CIERA Fellow Sasha Tchekovskoy, Assistant Professor of Physics and Astronomy at Northwestern’s Weinberg College of Arts and Sciences.

The hydrogen problem

FBOTs (pronounced F-bot) are a type of cosmic explosion initially detected in the optical wavelength. As their name suggests, transients fade almost as quickly as they appear. FBOTs reach peak brightness within a few days and then quickly fade – much faster than the rise and decay of standard supernovae.

After discovering FBOTs just four years ago, astrophysicists wondered if these mysterious events were related to another transient class: gamma-ray bursts (GRBs). The strongest and brightest outbursts on all wavelengths, GRBs are also associated with dying stars. When a massive star runs out of fuel and collapses into a[{” attribute=””>black hole, it launches jets to produce a powerful gamma ray emission.

Cocoon Around Jet From Dying Star

When a jet collides with collapsing layers of the star, it forms a ‘cocoon’ around the jet. Credit: Ore Gottlieb/Northwestern University

“The reason why we think GRBs and FBOTs might be related is because both are very fast — moving at close to the speed of light — and both are asymmetrically shaped, breaking the spherical shape of the star,” Gottlieb said. “But there was a problem. Stars that produce GRBs lack hydrogen. We don’t see any signs of hydrogen in GRBs, whereas in FBOTs, we see hydrogen everywhere. So, it could not be the same phenomenon.”

Using their new model, Gottlieb and his coauthors think they might have found an answer to this problem. Hydrogen-rich stars tend to house hydrogen in their outermost layer — a layer too thick for a jet to penetrate.


The cocoon envelops the jet and escapes with it from the star. As the cocoon cools, it releases heat as the emission of fast blue optical transients (FBOTs). Credit: Ore Gottlieb/Northwestern University

“Basically, the star would be too massive for the jet to pass through,” Gottlieb said. “So the jet will never exit the star, and that’s why it doesn’t produce GRB. Now, in these stars, the dying jet transfers all its energy to the cocoon, which is the only component to escape from the star. The cocoon will emit FBOT emissions, which will include hydrogen. This is another area where our model is fully consistent with all FBOT observations.

Assemble the image

Although FBOTs shine in optical wavelengths, they also emit radio waves and X-rays. Gottlieb’s model also explains them.

When the cocoon interacts with the dense gas surrounding the star, this interaction heats the stellar material to release a radio emission. And when the cocoon extends far enough from the black hole (formed from the collapsed star), X-rays can escape from the black hole. X-rays join radio and optical light to form a complete picture of the FBOT event.


A jet piercing the interior of a dying star. Credit: Ore Gottlieb/Northwestern University

While Gottlieb is encouraged by his team’s findings, he says more observations and models are needed before we can definitively understand the mysterious origins of FBOTs.

“When we calculated the amount of energy from the cocoon, it turned out to be as powerful as an FBOT.”
Ore Gottlieb, astrophysicist

“This is a new class of transients, and we know so little about them,” Gottlieb said. “We need to detect more of them earlier in their evolution before we can fully understand these explosions. But our model is able to draw a line between supernovae, GRBs and FBOTs, which I think is very elegant.”


A cocoon surrounding the jet of a collapsing star. As this cocoon escapes from the star, it cools, releasing heat in the form of fast blue optical transient emission (FBOT). Credit: Ore Gottlieb/Northwestern University

“This study paves the way for more advanced simulations of FBOTs,” Chekovskoy said. “This next-generation model will allow us to directly connect the physics of the central black hole to observables, allowing us to reveal the otherwise hidden physics of the central FBOT engine.”

Reference: “Shocked jets in CCSNe can power the zoo of fast blue optical transients” by Ore Gottlieb, Alexander Tchekhovskoy and Raffaella Margutti, April 11, 2022, Royal Astronomical Society Monthly Notices.
DOI: 10.1093/mnras/stac910

The study, “Shocked Jets in CCSNe May Power the Zoo of Fast Blue Optical Transients,” was supported by the National Science Foundation (Award Numbers AST-1815304 and AST-2107839). The authors developed the simulation using supercomputers at the Texas Advanced Computing Center at the University of Texas at Austin.

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