Back in August, an early release image from the James Webb Space Telescope revealed a bizarre sight: as many as 17 concentric rings encircling a binary star system, called Wolf-Rayet 140. Was it a spiral nebula, an alien megastructure or just an optical illusion?
The answer, revealed today, is dust. A new paper published in Nature Astronomy explains how stellar winds in this odd binary system blasts dust into near-perfect concentric circles every time the two stars come close to each other in their eccentric orbits.
“The stellar winds are the mechanical driving force,” co-author Dr. Patrick Morris, an astrophysicist from Caltech’s Infrared Processing and Analysis Center (IPAC) told Universe Today. “The collision of the stellar winds from both stars when they are close to each other, plus the orbital motion creates the arcs of dust, as we see them projected on the sky.”
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The exotic, odd-couple stars in WR-140 consist of an O-type star roughly 30 times the mass of the Sun and a Wolf-Rayet companion about 10 times the mass of the Sun, located just over 5,000 light-years from Earth. While other Wolf-Rayet systems form dust, none is known to make the nested dust shells like Wolf-Rayet 140 does.
These hemispheres or partial shells of dust in WR-140 were first detected in ground-based observations about twenty years ago.
“The earlier ground-based observations detected only a couple of inner shells and related features,” Morris said via email. “They interpreted them as dust formed during the last two (at that time) periastron passages of the massive O star and the more evolved carbon-rich Wolf-Rayet star.”
While that interpretation still holds, the new observations with JWST’s Mid-Infrared Instrument (MIRI) astounded the science team and revealed new details about the system.
“While the features were not unexpected, the number, spatial extent, and clarity of them certainly stunned all of us to the point that the team made intensive work of confirming their astrophysical reality of tracing over 130 years of dust production episodes, about once every 8 years.”
The new JWST observations, led by Ryan Lau, an astronomer at the National Science Foundation’s NOIRLab, reveals the unique ring pattern forms because the eccentric orbit of the stars in WR 140 is elongated, not circular. Only when the stars come close together every 7.93 years (coming about as close as Earth does to the Sun) the two stars’ solar winds collide, creating enough pressure where the gas emanating from the stars comes under sufficient pressure to form dust.
“The orbit is very eccentric, so the dust production conditions in the colliding stellar winds are most ideal during those periodic close approaches,” Morris explained.
Dust is one of the most important components of the Universe, as it plays a major role in the evolution of galaxies, and is an essential ingredient for new stars and planets. Since JWST is an infrared telescope, one of the basic uses of the new telescope is to learn more about the properties of dust in the universe, as it can provide snapshots of the contents, conditions and processes operating in galaxies – and even in other solar systems — at various stages of their evolution.
Morris feels that’s why WR-140 was targeted as an early observation for JWST.
“What probably ‘sold’ the proposal was part investigation of this class of massive stars that may be some of the first producers of dust in the early Universe,” Morris said, “and part altruistic motivation to test the MIRI instrument’s ability to detect faint emission in close proximity to a very bright source. The emission from the central binary is quite high in MIRI’s range of sensitivities (thus the obvious diffraction spikes in the raw images), while the shell emission is much weaker. This early experience is feeding back to the community of astronomers that will use MIRI.”
But even with the unique nature of WR-140, the researchers never imagined they’d be able to see the orbital physics of these two stars at work like this and with such clarity.
“For sure the clarity of the 17 or 18 partial shells caught me off guard,” Morris said, “and it implies that the winds of the WR and O star have ‘cleared out’ the surrounding interstellar medium over 1,000s of years so that those shells can freely (more or less) expand outward from the system. The surrounding area of Cygnus where WR140 resides is very busy with molecular gas and nearby star formation, so it really emphasizes the power these stars have to create their wind-driven bubbles.”
These distant observations can help astronomers understand the evolution of solar systems, including our own. Morris said there’s very good evidence that our solar system was formed in the swept-up gas and dust of such a bubble formed by a WR star, and meteorites found on Earth contain evidence of elements processed in the core of WR star.
“So, it’s likely our pre-solar nebula collapsed in the wind-blown material, and was chemically enriched by the products of the WR star’s core evolution!” Morris said. “I think with NASA’s new space telescope we’re going to learn a lot more about how these stars shape the material between stars and trigger new star formation in galaxies.”