This image from NASA's James Webb Space Telescope reveals a remarkable cosmic sight: at least 17 concentric dust rings emanating from a pair of stars. Located just over 5,000 light years from Earth, the duo is collectively known as Wolf-Rayet 140. Each ring was created when the two stars came close together and their stellar winds (streams of gas they blow into space) collided, compressing the gas and forming dust. The stars' orbits bring them together about once every eight years; like the rings of a tree's trunk, the dust loops mark the passage of time.
“We're looking at over a century of dust production from this system," said Ryan Lau, an astronomer at NSF's NOIRLab and lead author on a new study about the system, published in the journal Nature Astronomy. “The image also illustrates just how sensitive JWST is. Before, we were only able to see two dust rings, using ground-based telescopes. Now we see at least 17 of them."
In addition to Webb's overall sensitivity, the Mid-Infrared Instrument (MIRI) is uniquely qualified to study the dust rings, or what Lau and his colleagues call shells, because it sees in infrared light, a range of wavelengths invisible to the human eye.
The UK ATC played a key role in designing and building MIRI's spectrometer that was used to reveal the composition of the dust, formed mostly from the material ejected by the star which is a special type known as a Wolf-Rayet star.
A Wolf-Rayet star is born with at least 25 times more mass than our Sun and is nearing the end of its life. Burning hotter than in its youth, a Wolf-Rayet star generates powerful winds that push huge amounts of gas into space. The Wolf-Rayet star in this particular pair may have shed more than half its original mass via this process.
Forming Dust in the Wind
Transforming gas into dust is somewhat like turning flour into bread: It requires specific conditions and ingredients. The most common element found in stars, hydrogen, can't form dust on its own. But because Wolf-Rayet stars shed so much mass, they also eject more complex elements typically found deep in a star's interior, including carbon. The heavy elements in the wind cool down as they travel into space and are then compressed where the winds from both stars meet, like when two hands knead dough.
Some other Wolf-Rayet systems form dust, but none is known to make rings like Wolf-Rayet 140 does. The unique ring pattern forms because the orbit of the Wolf-Rayet star in WR 140 is elongated, not circular. Only when the stars come close together — about the same distance between Earth and the Sun — and their winds collide is the gas under sufficient pressure to form dust. Wolf-Rayet binaries that have circular orbits can produce dust continuously.
Lau and his co-authors think WR 140's winds also swept the surrounding area clear of residual material they might otherwise collide with, which may be why the rings remain so pristine.
Using MIRI's Medium Resolution Spectrometer's data, the new study provides the best evidence yet that Wolf-Rayet stars produce carbon-rich molecules of dust. And the preservation of the dust shells indicates that this dust can survive in the hostile environment between stars, going on to supply material for future stars and planets.
“Not only is this a spectacular image but this rare phenomenon reveals new evidence about cosmic dust and how it can survive in the harsh space environments," said Dr Olivia Jones, Webb Fellow at the UK ATC in Edinburgh, and a co-author of this new study. “These kinds of discoveries are only now opening up to us through the power of Webb and MIRI."
