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Bizarre secrets of a ticking star of the time bomb

Wolf-Rayet binary apep

Infrared image of the Wolf-Rayet track, nicknamed Apep, 8000 light years from Earth. Credit: European Southern Observatory

During the COVID blockade, an honors graduate student from the University of Sydney wrote a research paper on a star system dubbed one of the “exotic peacocks of the star world.”

Only one in a hundred million stars make the cut to be classified as Wolf-Rayet: hot, fiercely bright stars doomed to impending collapse in a supernova explosion, leaving only a dark remnant, such as a black hole.

The rarest of all, even among the Wolf-Rayets, are elegant binary pairs that, if conditions are right, are capable of pumping massive amounts of carbon dust pushed by their extreme stellar winds. As the two stars orbit each other, the dust is wrapped in a beautiful sooty and luminous tail. Only a handful of these sculpted spiral plumes have been discovered.

The subject of this study is the new star to join this elite club, but it has been found to break all the rules.

“Aside from the jaw-dropping image, the most remarkable thing about this star system is the way the expansion of its beautiful dust spiral has left us totally perplexed,” said Yinuo Han, who completed the research during his year of honor at the School of Physics.

“The dust seems to have a mind of its own, floating much slower than the extreme stellar winds that should guide it.”

Astronomers stumbled upon this conundrum when the system was discovered two years ago by a team led by Professor Peter Tuthill of the University of Sydney. This star system, 8000 light years from Earth, was named Apep after the serpentine Egyptian god of chaos.

Now Mr. Han’s research, published in Royal Astronomical Society Monthly Notices, confirms those results and reveals Apep’s bizarre physics in unprecedented detail.

Application of high resolution imaging techniques at the European Southern Observatory Very large telescope at Paranal in Chile, the team was able to probe the underlying processes that create the spiral we observe.

“The magnification required to produce the images was like seeing a chickpea on a table 50 kilometers away,” Han said.

Precise model

The team went beyond confirming the previous discovery, producing for the first time a model that matches the intricate spiral structure, improving scientists’ ability to understand the extreme nature of these stars.

Professor Peter Tuthill

Co-author Professor Peter Tuthill of the Sydney Institute for Astronomy at the University of Sydney. Credit: The University of Sydney

‘The fact that this relatively simple model can reproduce the spiral geometry at this level of detail is just fantastic,’ said Professor Tuthill.

However, not all physics is simple. Mr. Han’s team confirmed that the dust spiral is expanding four times slower than measured stellar winds, something unheard of in other systems.

The main theory to explain this bizarre behavior makes Apep a strong competitor to produce a gamma-ray burst when it finally explodes, something never seen before in the Milky Way.

Dr Joe Callingham, co-author of the study from the University of Leiden in the Netherlands, said: “There has been a flurry of research on Wolf-Rayet star systems – these are really the peacocks of the star world. The discoveries about these elegantly beautiful, but potentially dangerous objects are causing a real buzz in astronomy. “

He said this document is one of three that will be published this year on the Apep system only. Recently, the team showed that Apep was not just made up of one Wolf-Rayet star, but actually two. And colleagues from the Institute of Space and Astronautical Science in Japan will soon publish an article on another system, Wolf-Rayet 112. The lead author of that article, Ryan Lau, was a co-author of this article with Mr. Han .

Time bombs

Wolf-Rayet stars are massive stars that have reached their final stable phase before becoming supernova and collapsing to form compact remnants such as black holes or neutron stars.

“They are ticking time bombs,” said Professor Tuthill.

“In addition to displaying all the usual extreme Wolf-Rayets behavior, the main star of Apep appears to be spinning rapidly. That means he may have all the ingredients to detonate a long gamma-ray burst when he goes supernova.”

Gamma-ray bursts are among the most energetic events in the Universe. And they are potentially deadly. If a gamma-ray burst strikes Earth, it could strip the planet of its precious ozone layer, exposing us all to ultraviolet radiation from the Sun. Fortunately, Apep’s axis of rotation means it poses no threat to Earth.


The numbers reveal the extreme nature of Apep. The two stars are each about 10-15 times more massive than the Sun and over 100,000 times brighter. Where the surface of our natal star is around 5500 degrees, Wolf-Rayet stars are typically 25,000 degrees or more.

According to the team’s most recent findings, the huge stars in the Apep binary system orbit each other approximately every 125 years at a distance comparable to the size of our Solar System.

“The speeds of the stellar winds produced are simply mind blowing,” Han said. “They are spinning the stars at about 12 million kilometers per hour; is 1 percent of the speed of light.

“Yet the dust produced by this system is expanding much more slowly, at about a quarter the speed of the stellar wind.”

Mr. Han said the best explanation for this points to the rapidly rotating nature of the stars.

“It probably means that stellar winds are thrown in different directions at different speeds. The dust expansion we’re measuring is driven by slower winds launched near the star’s equator, “he said.

“Our model now fits quite well with the observed data, but we have not yet fully explained the physics of stellar rotation.”

Mr. Han will continue his astronomical studies at Cambridge University when he begins his doctorate later this year.

Reference: “The Apep Extreme Collision Wind System: Resolved Images of Center Track and Infrared Dust Plume” by Y Han, PG Tuthill, RM Lau, A Soulain, JR Callingham, PM Williams, PA Crowther, BJS Pope and B Marcote, 12 October 2020, Royal Astronomical Society Monthly Notices.
DOI: 10.1093 / mnras / staa2349

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