In our homes, dust is a nuisance. In space, it's a basic material from which stars, planets, and living things are made. Understanding where cosmic dust comes from is a ground-level question in astronomy, and researchers working with the JWST have uncovered one source: Wolf-Rayet stars.
Cosmic dust is made of tiny particles from nanometers to micrometers in size. These particles are the foundational building blocks for chemical evolution, the formation of complex organic molecules, and even play critical roles in star formation. There are different varieties of cosmic dust, including silicate dust, carbon-based dust, metallic grains containing iron or nickel, and even ice-covered particles in the coldest regions of space.
Cosmic dust is a backbone material, and astronomers have several questions they're actively asking. They want to know where dust came from in the early Universe, when the very first stars were forming and only beginning to create heavy elements. How do dust grains grow in space? How do they survive space's harsh conditions? How do dust grains stick together in the early phases of planet formation?
Carbon dust is of particular interest in all this because life is carbon-based, at least on Earth, and so it plays a critical role in interstellar chemistry and chemical evolution. The surfaces of carbon dust grains are chemically active and things like water, ammonia, and methanol collect on their surfaces and allow chemical reactions that create complex organic compounds.
The importance of simple dust can't be overstated, and astronomers have long wondered where it comes from, especially the all important carbon dust. New research in The Astrophysical Journal shows how Wolf-Rayet (WR) stars create dust and spread it out into the interstellar medium (ISM). It's titled "Carbon-rich Dust Injected into the Interstellar Medium by Galactic WC Binaries Survives for Hundreds of Years," and the lead author is Dr. Noel Richardson, an associate professor of Physics and Astronomy at Embry‑Riddle Aeronautical University.
Wolf-Rayet stars are an unusual class of star. They're extremely hot and their temperatures range from 30,000 to 200,000 Kelvin, meaning they're some of the hottest stars we know of. They're also extremely luminous, and can outshine the Sun by millions of times. They have masses between about 10 and 25 solar masses, and may have been more massive when they first formed. They WR stars also emit extremely powerful stellar winds that reach velocities of 5,000 km/second. That's a million times faster than the Sun's wind.
These extreme objects are evolved, and WR stars represent stars in a brief but important phase of their life. Their outer hydrogen is gone, consumed in nuclear fire, and they fuse helium and heavier elements in their cores. They're basically near the end, poised to be destroyed in massive supernova explosions, or to collapse in on themselves and form stellar-mass black holes.
“Wolf-Rayet stars are essentially highly evolved massive stars that don’t show hydrogen at all,” said lead author Richardson in a press release. “They’ve lost their hydrogen in the outer part of the star, fusing helium in their core, which means they are nearing the end of their life cycle.”
All WR stars unleash viciously powerful stellar winds, and previous research shows that for at least one Wolf-Rayet star, the winds collide with the winds of another star and condense into carbon dust. That star is called WR-140. Astronomers think that WR140 is the prototypical cosmic dust producer.
Dust around WR stars represents a paradox. The intense outward radiation pressure from the stars should simply blow them away, yet the powerful winds are necessary for them to form.
In their research, Richardson and his co-researchers examined four WR stars. They're a sub-type of WR star called WCd stars, carbon-rich stars known for excess infrared emissions from surrounding dust. The stars in their sample are either in confirmed binary systems or known to undergo the periodic creation of dust.
“Not only did we find that the dust in these systems is long-lived and making its way out into space, we discovered this is not unique to just one system,” said Richardson.
One of the study's authors is Dr. Ryan Lau from the NSF's NOIRLab. Lau was involved with the earlier research that used the JWST to observe WR-140, the only previously-known WR star to have a dust ring. Lau commented on the new observations, saying that, “It confirmed that we are seeing the same pattern of surviving dust shells that we did around WR-140 in other systems,” said Lau. “These observations show that the dust produced by Wolf-Rayet stars can survive the harsh stellar environment.”
The new research confirms that in stellar pairs, winds from a WC star collides with its companion's winds and forms carbon dust. "As this dust is carried toward the interstellar medium (ISM) at close to the WCd wind speed and the binary continues through its orbit, a spiral structure forms around the system," the authors write in their research. The shape of this dusty structure depends on orbits, mass-loss rates, and wind speeds.
When WR-140 was observed, astronomers detected 17 concentric dust shells surrounding the binary. In these new images, the four stars also featured concentric rings, a telltale sign that the creation of dust is episodic. "Regular spacing of dust features confirms the periodic nature of dust formation, consistent with a connection to binary motion," the authors explain.
Not only that, but the observations show that the dust is long-lived, considering the intense environment. "In this analysis, we show that the dust is long-lived, detected with an age of at least 130 yr, but more than 300 yr in some systems," the researcher write.
The team was also able to determine the motion of the dust, showing that it's roughly in line with the wind speed. "We use these images to estimate the proper motion of the dust, finding the dust to propagate out to the ISM with motion comparable to the wind speed of the WC stars," they write.
The JWST observations also show some unexpected shapes similar to proplyds around one of the four stars in their sample. "In addition to these results, we observe unusual structures around WR 48a, which could represent dusty clumps shaped by photoevaporation and wind ablation like young proplyd objects," the researchers explain. These structures demonstrate that the dust could be long-lived, and "should be accounted for in galactic dust budgets."
If WR stars can create dust that can endure for centuries, astronomers will need to reconsider how they think about dust and what role it plays in star formation.
While this work soothes some of astronomers' gnawing curiosity about cosmic dust, there's much more work to be done. “Where does this dust go?” Lau asked. “We want to learn what exactly the chemistry of this dust is. To do that, we need to take spectra to identify specific grain composition — the physical properties — to get an idea of the chemical contribution to the interstellar medium.”