From stars to the laboratory: how a speck of dust gives us insight into processes in an exploding star

While novae are traditionally studied using observations from space or earth-based telescopes, the laboratory study of a speck of stardust provided insight into the conditions of dust formation in nova ejecta. In this paper, we report the identification of a silicate-oxide inclusion inside of a graphite spherule that originated in a nova outburst. The coexistence of carbon-rich and oxygen-rich material indicates that C and O can co-condense in the same circumstellar environment.

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Apr 29, 2019

As the French novelist, Victor Hugo once wrote: “Where the telescope ends, the microscope begins. Which of the two has the grander view?” (Les Misérables, 1890). While space or ground-based telescopes are traditionally used to study stellar and interstellar environments, the laboratory study of grains, called stardust or presolar grains, identified in extraterrestrial materials (e.g., meteorites) has opened up a new field in astronomy and astrophysics, providing direct ground-truth information on individual stars. The analysis of isotopic and chemical compositions and microstructure of individual presolar grains using ultrahigh-resolution ion mass spectrometry and electron microscopy techniques provide us with a snapshot of the thermodynamic conditions (e.g., pressure and temperature) and nuclear processes happening in their parent star at the time of their condensation. Presolar grains formed over 4.56 billion years ago in circumstellar envelops or stellar ejecta and were preserved inside in the fine-grained material of unequilibrated and minimally altered planetary bodies, such as asteroids and comets.

How small is small?
Comparison of the relative size of presolar graphite LAP-149 with other objects. The Figure is not to scale.

Most stardust grains identified thus far originated from asymptotic giant brand or red giant stars and supernova explosions. A few grains also appear to have originated in the ejecta of nova outburst, a type of binary star system where the remnant core of a 'dead' star, called a white dwarf, captures material from the envelope of its nearby companion star (typically a main sequence star) until the accreted hydrogen undergoes rapid fusion, triggering the nova explosion.

Astronomical observations of nova ejecta suggest that they are prolific dust producers with the possible concurrent condensation of both C- and O-rich dust within the same ejecta. They also showed that nova outbursts appear asymmetric and heterogeneous with the presence of chemically-distinct clumps of dust in the ejecta.

False-color composite image of GK Persei nova
False-color composite image of GK Persei nova. X-rays from NASA Chandra X-ray Observatory (blue), optical data from NASA's Hubble Space Telescope (yellow), and radio data from the National Science Foundation's Very Large Array (pink). The optical data in yellow shows distinct clumps of materials in the ejecta around the white dwarf. Image credits: NASA Chandra X-ray Observatory/Harvard University/

However, it was traditionally believed that C- and O-rich dust grains form under different conditions in stellar environments, with carbonaceous grains condensing in C-rich environments, and silicate and oxide grains in O-rich environments. Furthermore, no laboratory evidence of co-condensation of carbonaceous and O-rich grains had been identified in presolar grains.

Using high-resolution state-of-the-art microscopes, we identified an O-rich inclusion, composed of nanocrystalline silicate and oxide, inside of a presolar graphite grain from a nova outburst. Our paper provides laboratory evidence of the concurrent condensation and large-scale mixing of silicate and carbonaceous dust between different clumps of dust within the nova ejecta.

Illustration of dust clumps in a nova
Illustration of dust clumps in a nova. Billions of years ago, before our solar system was born, a dead star known as a white dwarf in a nearby binary star system accumulated enough material from its companion to cause it to "go nova." The stellar explosion forged dust grains with exotic compositions not found in our solar system. A team of researchers led by the UA found such a grain (inset image), encased in a meteorite, that survived the formation of our solar system and analyzed it with instruments sensitive enough to ID single atoms in a sample. Measuring one 25,000th of an inch, the carbon-rich graphite grain (red) revealed an embedded speck of oxygen-rich material (blue), two types of stardust that were thought could not form in the same nova eruption. Illustration: University of Arizona/Heather Roper

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Pierre Haenecour

Assistant Professor (Fall 2019), The University of Arizona

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