The findings are published in the journal Nature and were obtained using infrared photos of the binary star system WR140 acquired over a 16-year period.
In a related investigation of WR140, which was reported in Nature Astronomy, NASA’s James Webb Space Telescope (JWST) was able to see considerably deeper and capture an image of not just one accelerating dust cloud but over 20 of them, nestled inside one another like a massive set of onion skins.
A massive Wolf-Rayet star and an even more massive blue supergiant star are gravitationally locked in an eight-year orbit in the star WR140. With one of the largest optical telescopes in the world at the Keck Observatory in Hawaii, this binary star in the Cygnus constellation has been observed for 20 years.
WR140 periodically exhales dust plumes that are thousands of times longer than the distance between the Earth and the Sun. Every eight years, these dust plumes appear, providing astronomers with a rare chance to study the effects of starlight on the matter.
It is well known that light has momentum and pushes matter with this radiation pressure. Astronomers frequently observe the result of this occurrence in the form of matter traveling through space at a great speed, but it has proved challenging to observe the phenomenon itself. In a stellar setting like this, direct recording of acceleration caused by forces other than gravity is extremely unusual.
“It’s hard to see starlight causing acceleration because the force fades with distance, and other forces quickly take over,” said Yinuo Han from Cambridge’s Institute of Astronomy, the first author of the Nature paper. “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. WR140 is a binary star whose ferocious radiation field supercharges these effects, placing them within reach of our high-precision data.”
Although stellar winds are produced by all-stars, those from Wolf-Rayet stars can sometimes resemble stellar hurricanes. Wind-borne substances like carbon condense out as soot, which is still hot enough to light intensely in the infrared. This provides telescopes with something to look at, similar to smoke in the breeze.
It’s hard to see starlight causing acceleration because the force fades with distance, and other forces quickly take over. 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. WR140 is a binary star whose ferocious radiation field supercharges these effects, placing them within reach of our high-precision data.
Yinuo Han
The scientists recovered sufficiently fine photos of WR140 for the study using interferometry, an imaging technique that can act as a zoom lens for the 10-meter Keck telescope mirror.
Han and his crew discovered that the star’s dust does not stream out from it in a hazy ball with the wind. Instead, the dust originates on the surface of a cone-shaped shock front between the two stars, when the winds from the two stars collide.
The shock front revolves as a result of the continual motion of the orbiting binary star. Similar to how water droplets make a spiral in a lawn sprinkler, the sooty plume spirals around itself.
The researchers found that WR140 has other tricks up its sleeve. Dust production increases and decreases as the binary approaches and departs the point of the closest approach since the two stars are not on circular orbits but rather elliptical ones. The astronomers were able to determine the location of dust features in three-dimensional space by modeling these effects into the dust plume’s three-dimensional geometry.
“Like clockwork, this star puffs out sculpted smoke rings every eight years, with all this wonderful physics written then inflated in the wind like a banner for us to read,” said co-author Professor Peter Tuthill from the University of Sydney. “Eight years later as the binary returns in its orbit, another appears the same as the one before, streaming out into space inside the bubble of the previous one, like a set of giant nested Russian dolls.”
The researchers had a unique laboratory to study the acceleration zone since the dust created by this Wolf-Rayet is so predictable and expands to such great distances.
“In the absence of external forces, each dust spiral should expand at a constant speed,” said Han, who is also a co-author on the JWST paper. “We were puzzled at first because we could not get our model to fit the observations, until we finally realised 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.”
“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,” said Tuthill. “When I look at the data now, I see WR140’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.”
With JWST now in operation, researchers can learn much more about WR140 and similar systems.
“The Webb telescope offers new extremes of stability and sensitivity,” said Ryan Lau who led the JWST study. “We’ll now be able to make observations like this much more easily than from the ground, opening a new window into the world of Wolf-Rayet physics.”
The research was funded in part by the Gates Cambridge Trust.
