NASA’s James Webb Space Telescope managed to take a picture of a star of the Wolf-Rayet 140 duo and the 17 dust rings surrounding them, and the result is simply stunning.
The system, known as WR 140, consists of two massive stars. One of them is an O-type star, the largest and hottest class of stars we know of. The second is called Wolf-Rayet, a class of massive stars that are so energetic that they eject most of their hydrogen and often throw heavier elements such as carbon or nitrogen into their environment. These two giants orbit close enough to each other to make one revolution in less than eight years, although the orbit is quite eccentric.
Because of their high-energy nature, both of these stars have powerful stellar winds that eject a lot of matter into the surrounding space. And where their winds interact, the pressure can become high enough for the winds to condense into dust particles. Due to orbital dynamics, this results in a partial ring-like structure, with most of the condensation occurring on one side of the binary. Observations using ground-based telescopes have revealed one or two rings extending to a distance of about 5,000 times the Earth-Sun distance (called an astronomical unit, or AU).
These structures looked interesting enough to make WR140 a target for Webb’s Discretionary Early Release science program, which was developed by Webb’s director. And the images are truly exceptional, showing a series of up to 17 rings that extend about a light-year from the binary stars. There are probably additional rings that extend even further, but they are too faint to be detected.
The rings are not perfectly even, and some areas are thicker than others. These differences are superimposed on the diffraction peaks caused by the shape of the Webb mirrors and some associated equipment. But the diffraction peaks affect all wavelengths, while the ring features are much brighter at lower infrared energies, allowing them to be distinguished.
In theory, the details of the rings, including their asymmetry, are controlled by the orbital interactions of the two stars. So the research team built a model that includes orbital details and the winds produced by the two stars. The match between the model results and what is actually seen in HR 140 is astonishingly good, in the sense that it accurately predicts many fine details of the ring system. This suggests that we have a good understanding of the physical forces that create this system, including the factors that create more dust at some points in the orbit than at others.
Assuming that one ring forms per orbit, the 17 current rings indicate that they form in about 130 years. Since they now span about a light year, we can conclude that they are moving away from the binary stars at about 2,600 kilometers every second.
Despite some irregularities, the expansion is surprisingly uniform, given that the material in the rings is subject to alternating gravitational pull, radiation pressure from the stars, and drag from all the other material that has been ejected by the stars.
The Wolf-Rayet star in WR 140 is rich in carbon, which means that much of the dust present in the rings must be correspondingly rich in organic chemicals. And thanks to Webb’s equipment, we can confirm it. The chemical mixture appears very complex, with no individual molecules distinguishable from the spectrum. But it appears to be rich in ringed, benzene-like structures called polycyclic aromatic hydrocarbons. They tend to form in carbon-rich, hydrogen-poor environments — just what you’d expect from a carbon-rich Wolf-Rayet star.
Since both physics and chemistry behave exactly as expected, there aren’t that many mysteries to solve. The information is useful for other areas of astronomy because dust in the interstellar medium is a major contributor to the formation of stars and planets, and these stars can help us understand how some of it got there.
But that doesn’t mean you shouldn’t spend some time marveling at the fact that relatively easy-to-understand chemistry and physics can produce the truly stunning structures that Webb discovered.