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Dying stars could seed the interstellar medium with carbon nanotubes

By Daniel Stolte, University Communications


In this image of the Spirograph Nebula, a dying star about 2,000 light-years from Earth, NASA’s Hubble Space Telescope has revealed remarkable textures traversing the star’s envelope of dust and gas. The Arizona researchers have now found evidence that complex carbon nanotubes could be forged in such environments.
NASA and the Hubble Heritage team (STScI/AURA)

In the mid-1980s, the discovery of complex carbon molecules drifting through the interstellar medium attracted considerable attention, perhaps the most famous examples being Buckminsterfullerene, or “buckyballs” – spheres composed of 60 or 70 atoms of carbon. However, scientists are struggling to understand how these molecules can form in space.

In a paper accepted for publication in the Journal of Physical Chemistry A, researchers from the University of Arizona suggest a surprisingly simple explanation. After exposing silicon carbide – a common ingredient of dust grains in planetary nebulae – to conditions similar to those found around dying stars, the researchers observed the spontaneous formation of carbon nanotubes, which are molecules shaped of highly structured rods made of several layers of carbon sheets. . The findings were presented June 16 at the 240th meeting of the American Astronomical Society in Pasadena, California.

Led by a researcher from UArizona Jacques Bernalwork is based on research published in 2019, when the group showed they could create buckyballs using the same experimental setup. The work suggests that buckyballs and carbon nanotubes could form when silicon carbide dust produced by dying stars is hit by high temperatures, shock waves and high-energy particles, leaching silicon from the surface and leaving carbon behind.

The results support the idea that dying stars could seed the interstellar medium with nanotubes and possibly other complex carbon molecules. The findings have implications for astrobiology, as they provide a mechanism for concentrating carbon that could then be transported to planetary systems.

“We know from infrared observations that buckyballs inhabit the interstellar medium,” said Bernal, a postdoctoral research associate at UArizona. Lunar and planetary laboratory. “The big problem has been explaining how these massive, complex carbon molecules could possibly form in a hydrogen-saturated environment, which is what you typically have around a dying star.”

Schematic illustration of a silicon carbide grain releasing carbon nanotubes

In this schematic illustration, experimental heating causes a grain of silicon carbide to shed silicon atoms (green). This leaves carbon (black) atoms behind, which assemble into sheets of graphene and eventually into rod-shaped carbon nanotubes and spherical buckyballs.
Jacob Bernal/University of Arizona

The formation of carbon-rich molecules, let alone carbon-only species, in the presence of hydrogen is virtually impossible due to thermodynamic laws. The study’s new findings offer an alternative scenario: Instead of assembling individual carbon atoms, buckyballs and nanotubes could result from a simple rearrangement of the structure of graphene – single-layer carbon sheets that are known to form on the surface of heated silicon carbide grains.

This is exactly what Bernal and his co-authors observed when they heated commercially available silicon carbide samples to temperatures occurring in dying or dead stars and imaged them. As the temperature approached 1,050 degrees Celsius, small hemispherical structures with an approximate size of about 1 nanometer were observed on the surface of the grain. Within minutes of continuous heating, the spherical buds began to grow into rod-like structures, containing multiple layers of graphene with curvature and dimensions indicative of a tubular shape. The resulting nanotubules ranged from about 3 to 4 nanometers in length and width, larger than buckyballs. The largest specimens imaged were composed of more than four layers of graphitic carbon. During the heating experiment, the tubes were observed to wiggle before budding from the surface and being sucked into the vacuum surrounding the sample.

“We were surprised to be able to make these extraordinary structures,” Bernal said. “Chemically, our nanotubes are very simple, but they are extremely beautiful.

Named after their resemblance to the architectural works of Richard Buckminster Fuller, fullerenes are the largest molecules currently known to exist in interstellar space, which for decades was thought to be devoid of any molecule containing more than a few atoms, 10 or so. more. It is now well established that the C60 and C70 fullerenes, which contain 60 or 70 carbon atoms respectively, are common ingredients of the interstellar medium.

One of the first of its kind in the world, the transmission electron microscope housed at the Kuiper Materials Imaging and Characterization Facility in Arizona is uniquely suited to simulate the planetary nebula environment. Its 200,000 volt electron beam can probe matter up to 78 picometers – the distance of two hydrogen atoms in a water molecule – allowing individual atoms to be seen. The instrument operates in a vacuum closely resembling the pressure – or lack thereof – thought to exist in circumstellar environments.

While a spherical molecule of C60 measures 0.7 nanometers in diameter, the nanotube structures formed in this experiment were many times the size of C60, easily exceeding 1,000 carbon atoms. The study authors are confident that their experiments accurately replicated the temperature and density conditions that would be expected in a planetary nebula, the co-author said. Lucy ZiurysArizona Regents Professor of Astronomy, Chemistry and Biochemistry.

“We know the raw material is there, and we know the conditions are very close to what you would see near the envelope of a dying star,” she said. “There are shock waves going through the envelope, so temperature and pressure conditions have been shown to exist in space. We also see buckyballs in these planetary nebulae – in other words, we let’s see the beginning and end products you expect in our experiments.”

These experimental simulations suggest that the carbon nanotubes, along with the smaller fullerenes, are then injected into the interstellar medium. Carbon nanotubes are known to have high stability against radiation, and fullerenes are able to survive for millions of years when adequately shielded from high-energy cosmic radiation. Carbon-rich meteorites, such as carbonaceous chondrites, could also contain these structures, the researchers propose.

According study co-author Tom Zegaa professor at the UArizona Lunar and Planetary Lab, the challenge is to find nanotubes in these meteorites, due to the very small grain size and because meteorites are a complex mixture of organic and inorganic materials, some of which have sizes similar to those of nanotubes.

“Nevertheless, our experiments suggest that such materials could have formed in interstellar space,” Zega said. “If they survived the journey to our local part of the galaxy where our solar system formed around 4.5 billion years ago, then they could be preserved inside the remaining material.”

Zega said a prime example of such remains is Bennu, a carbonaceous near-Earth asteroid whose NASA UArizona-led OSIRIS-REx mission recovered a sample in October 2020. Scientists are eagerly awaiting the arrival of this sample, scheduled for 2023.

“Asteroid Bennu could have retained these materials, so it’s possible we could find nanotubes there,” Zega said.

Funding for this work was provided by NSF Grant AST-2137919, NSF Grant AST-1907910, and NASA grants 80NSSC19K0509 and 80NSSC21K0593 (“Alien Earths”). Bernal received an Ascend Fellowship in Mathematical and Physical Sciences from the National Science Foundation.

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