Rare Star System Gives Insights into the Origins of Carbon Dust in the Galaxy

A detailed new image from NASA's James Webb Space Telescope (JWST) has unveiled the most precise portrait yet of a rare cosmic system: a pair of massive dying stars, known as Wolf–Rayet stars, orbiting tightly around each other. The rare system, named Apep after an ancient Egyptian deity that shares a similar shape, was first imaged in 2016 by the Very Large Telescope in Chile. The new JWST image zooms out to capture the concentric spherical waves of dust that emanate from the star system, giving insight to the primordial origins of the carbon building blocks that could ultimately contribute to forming planets like our own.

The findings are described in a paper published in the Astrophysical Journal on November 19. Caltech postdoctoral scholar Yinuo Han is the study's first author.

Our galaxy contains hundreds of billions of stars, only about 1,000 of which are Wolf–Rayet stars. These are more than 20 times as massive as our Sun and are much further along in the stellar life cycle. They are extremely bright and thus easy to spot with telescopes, but because they represent only a short-lived phase of the most massive stars' lifespan, they are also rare.

"Out of the hundreds of billions of stars in the Milky Way, Apep is the only system we know of with a configuration like this," Han says. "One Wolf–Rayet star is rare enough, but in Apep there are two stars of this kind dancing together in a binary."

As the two Wolf–Rayet stars orbit each other, they violently emit particles in the form of "stellar winds." While our Sun also emits stellar wind, the Apep system's wind has far more mass and significantly greater speed, making it more than a billion times more powerful than our Sun's.

When the winds from the two stars collide, the particles can clump together, forming dust. In particular, the dust in the Apep system is rich in amorphous carbon, because hydrogen, which is abundant in typical stars, was dissipated earlier in the stars' evolution. Due to the presence of this carbon, which retains heat, the dust shells remain warm enough to be imaged in infrared light. JWST's Mid-Infrared Instrument (MIRI) was able to image details of the shells' concentric patterns, which spiral outward from the stars with almost clock-like regularity—producing a cosmic fossil record of how the stars' winds have interacted with one another over centuries.

"Despite the energetic collision of the stellar winds from which dust forms, the dust expands predictably as highly regular shells," Han explains. "This not only reflects how stable the stars' orbits have been but also enables us to make precise measurements of the wind speed, dust temperature, and orbital motion."

The dust shells extend at least tens of trillions of kilometers from their stars and contribute a significant amount of carbon into the galaxy, as if the stars are fertilizing the Milky Way with the building blocks that could go on to become planets.

The regularity of the dust shells is not the only notable part of the new image. The detailed JWST image also shows that a third star, a supergiant O-type star, appears to be carving out a hole in the dust shell, possibly with its own stellar wind. In a related paper in The Astrophysical Journal, Macquarie University graduate student Ryan White describes a new mathematical model that accurately simulates the system's geometry, indicating that the third star is indeed part of the Apep system.

Researchers are still working to determine the system's exact distance from Earth, currently predicted to be 15,000 light-years. Knowing the distance would enable scientists to more precisely measure the dust's expansion speed and confirm whether the winds vary in strength by direction—a clue that could reveal if Apep is a progenitor of gamma-ray bursts, some of the most energetic events in the universe.

Additionally, the JWST images show a strange filament of dust slicing through the system that may be caused by an especially powerful ejection of material. The filament will be a focus of future study.

The paper is titled "The formation and evolution of dust in the colliding-wind binary Apep revealed by JWST." In addition to Han and White, co-authors are Joseph R. Callingham of the Netherlands Institute for Radio Astronomy and the University of Amsterdam, Ryan M. Lau of Caltech's IPAC, Benjamin J. S. Pope of Macquarie University and the University of Queensland in Australia, Noel D. Richardson of Florida's Embry-Riddle Aeronautical University, and Peter G. Tuthill of the University of Sydney. Funding was provided by NASA, the Space Telescope Science Institute, the Andy Thomas Space Foundation, the Australian Research Council, and the Research Corporation for Science Advancement.