An image of the binary system WR 140, captured by the NASA/ESA/CSA James Webb Space Telescope in July 2022, has baffled astronomers worldwide — even triggering a speculation that it might be evidence of an alien megastructure. But in two new studies, astronomers explain that the 17 concentric rings around WR 140 are actually a series of dust shells created by the interaction between a pair of hot stars.
WR 140 is a binary system located approximately 6,065 light-years away in the constellation of Cygnus.
Also known as HD 193793, HIC 100287 or IRAS 20187+4341, the system is comprised of a huge Wolf-Rayet star and an even bigger blue supergiant star, gravitationally bound in a 7.93-year orbit.
WR 140 episodically puffs out plumes of dust stretching thousands of times the distance from the Earth to the Sun.
These dust plumes, produced every eight years, give astronomers an unique opportunity to observe how starlight can affect matter.
“Like clockwork, WR 140 puffs out a sculpted smoke ring every eight years, which is then inflated in the stellar wind like a balloon,” said Professor Peter Tuthill, an astronomer in the Sydney Institute for Astronomy at the University of Sydney.
“Eight years later, as the binary returns in its orbit, another ring appears, the same as the one before, streaming out into space inside the bubble of the previous one.”
Because the two stars are in elliptical rather than circular orbits, dust production turns on and off as WR 140’s binary companion nears and then departs the point of closest approach.
Based on data collected with other telescopes since 2006, Professor Tuthill and colleagues created a 3D model of the dust plume’s geometry. That model turned out to perfectly explain the bizarre results obtained by Webb in July 2022.
What’s more, the astronomers showed direct evidence of intense starlight driving into matter and accelerating it, after tracking titanic plumes of dust generated by the violent interactions between two colossal stars over 16 years.
It’s known that starlight carries momentum, exerting a push on matter known as ‘radiation pressure.’
Astronomers often see the aftermath of this in the form of matter coasting at high speed around the cosmos, but have never caught the process in the act.
Direct observation of acceleration due to forces other than gravity is rarely witnessed, and never in a stellar environment like this.
“It’s hard to see starlight causing acceleration because the force fades with distance, and other forces quickly take over,” said Dr. Yinuo Han, an astronomer in the Institute of Astronomy at the University of Cambridge.
“To witness acceleration at the level that it becomes measurable, the material needs to be reasonably close to the star or the source of the radiation pressure needs to be extra strong.”
“WR 140 is a binary star whose ferocious radiation field supercharges these effects, placing them within reach of our high-precision data.”
The authors discovered that the dust does not stream out from the star with the wind forming a hazy ball, as had been thought.
Instead, the dust condenses adjacent to where the winds from the two stars collide, on the surface of a cone-shaped shock front between them.
Because the orbiting binary star is in constant motion, the shock front also rotates.
The sooty plume gets wrapped into a spiral, in the same way that droplets form a spiral in a garden sprinkler.
“In the absence of external forces, each dust spiral should expand at a constant speed,” Dr. Han said.
“We were puzzled at first because we could not get our model to fit the observations until we finally realized that we were seeing something new.”
“The data did not fit because the expansion speed wasn’t constant, but rather that it was accelerating. We’d caught that for the first time on camera.”
Once they added the acceleration of dust by starlight into their 3D model of the WR 140 binary, it explained their observational data perfectly.
And also ended up explaining the strange concentric rings later spotted with Webb.
“In one sense, we always knew this must be the reason for the outflow, but I never dreamed we’d be able to see the physics at work like this,” Professor Tuthill said.
“When I look at the data now, I see WR 140’s plume unfurling a like giant sail made of dust. When it catches the photon wind streaming from the star, like a yacht catching a gust, it makes a sudden leap forward.”
The results appear in two papers in the journal Nature and the journal Nature Astronomy.
Y. Han et al. 2022. Radiation-driven acceleration in the expanding WR 140 dust shell. Nature 610, 269-272; doi: 10.1038/s41586-022-05155-5
R.M. Lau et al. Nested dust shells around the Wolf-Rayet binary WR 140 observed with JWST. Nat Astron, published online October 12, 2022; doi: 10.1038/s41550-022-01812-x
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