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.
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.
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.
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.