Using NASA’s James Webb Space Telescope (Webb) together with data from the ground based Very Large Telescope (VLT) in Chile, an international astronomy team has uncovered extraordinary images of a rare stellar system called Apep, showing four distinct dust shells spiralling outward from three massive stars locked in a cosmic dance.
Spiralling dust shells expand outward from the Apep star system, named after the ancient Egyptian serpent god of chaos, in this JWST image, overlaid by lines mapping each shell’s exact shape. Credit: JWST / reduced and processed by S Dholakia and B J S Pope
‘Wolf-Rayet’ stars are a rare class of massive binary stars, where the earliest carbon in the universe is forged. The discovery helps astronomers understand how these stars interact and evolve over centuries.
Macquarie University Master of Research student Ryan White has refined the orbit of the Wolf-Rayet stars in the Apep system in a paper published this week in The Astrophysical Journal. He combined precise measurements of the ring location from Webb’s image with the speed of the shells’ expansion from observations taken by the VLT over eight years.
“This is a one-of-a-kind system with an extremely long orbital period,” White said. “The next longest orbit for a dusty Wolf-Rayet binary is about 30 years. Most have orbits between two and 10 years.”
White’s paper was published simultaneously in The Astrophysical Journal with another paper out of Caltech in Pasadena, California, with lead author Dr Yinuo Han.
“Looking at Webb’s new observations was like walking into a dark room and switching on the light – everything came into view,” said Dr Han.
“There is dust everywhere in Webb’s image, and the telescope shows that most of it was cast off in repetitive, predictable structures.”
How the discovery unfolded
Webb observations delivered a first of its kind: a crisp mid-infrared image of a system of four serpentine spirals of dust, one expanding beyond the next in precisely the same pattern. (The fourth is almost transparent, at the edges of the Webb image.)
Observations of this system taken prior to Webb had only detected one shell, and while the existence of outer shells was hypothesised, ground-based telescopes were unable to uncover any.
The Webb image combined with several years of data from the European Southern Observatory’s Very Large Telescope (VLT) in Chile narrowed down how often the pair swing by one another: once every 190 years.
Over each incredibly long orbit, the stars pass closely for 25 years and form dust.
Webb observations also confirmed that there are three stars gravitationally bound to one another in this system. The dust ejected by the two Wolf-Rayet stars is slashed by a third star, a massive supergiant, which carves a hole into each expanding cloud of dust from its wider orbit. (All three stars form a single bright point of light in Webb’s image.)
Associate Professor Ben Pope (pictured) co-supervised the research that unlocked the mysteries of the rare Apep triple-star system, led by Master of Research student Ryan White.
Han, White, and their co-authors refined the Wolf-Rayet stars’ orbit by combining precise measurements of the ring location from Webb’s image with the speed of the shells’ expansion from observations taken by the VLT over eight years.
High-speed ‘skirmish’
The dust-producing Wolf-Rayet stars in Apep aren’t exactly on a tranquil cruise. They are whipping through space and sending out dust at 2,000 to 3,000 kilometres per second.
When the two Wolf-Rayet stars approach and pass one another, their strong stellar winds collide and mix, forming and casting out heaps of carbon-rich dust for a quarter century at a time. In similar systems, dust is shot out over mere months, like the shells in the system Wolf-Rayet 140 .
That dust is also very dense. The specific makeup of the dust is another reason why Webb was able to observe so much more: it largely consists of amorphous carbon.
“Carbon dust grains retain a higher temperature even as they coast far away from the star,” Dr Han said.
While the exceptionally tiny dust grains are considered warm in space, the light they emit is also extremely faint, which is why it can only be detected from space by Webb’s MIRI (Mid-Infrared Instrument).
Slicing dust
“I was shocked when I saw the updated calculations play out in our simulations,” he said. “Webb gave us the ‘smoking gun’ to prove the third star is gravitationally bound to this system.” Researchers have known about the third star since the VLT observed the brightest innermost shell and the stars in 2018, but Webb’s observations led to an updated geometric model, clinching the connection.
“We solved several mysteries with Webb,” Dr Han said. “The remaining mystery is the precise distance to the stars from Earth, which will require future observations.”
Future of Apep
The two Wolf-Rayet stars were initially more massive than their supergiant companion but have shed most of their mass. It’s likely that both Wolf-Rayet stars are between 10 and 20 times the mass of the Sun, and that the supergiant is 40 or 50 times as massive as the Sun.
Eventually, the Wolf-Rayet stars will explode as supernovae, quickly sending their contents into space. Either may also emit a gamma-ray burst, one of the most powerful events in the universe, before possibly becoming a black hole.
Wolf-Rayet stars are very rare in the universe. Only a thousand are estimated to exist in our Milky Way galaxy, which contains hundreds of billions of stars overall. Of the few hundred Wolf-Rayet binaries that have been observed to date, Apep is the only example that contains two Wolf-Rayet stars of these types in our galaxy; most only have one.