Presolar Graphitic Carbon Spherules: Rocks from Stars

Abstract

Graphitic carbon spherules found in primitive meteorites have large carbon isotope anomalies, indicating that they are carbonaceous stardust (also known as presolar grains) expelled from dying stars prior to the formation of the Sun. Presolar spherules show varying degrees of graphitization, ranging from poorly graphitic, turbostratic layers in low-density spherules to well-crystallized graphitic outer shells in high-density ones, and some spherules also contain a polycrystalline phase in their core. Within the spherules, grains of other refractory phases (including carbides and metals) are common, and these assemblages can be studied as one would study a rock. The isotopic and microstructural information available from these presolar graphitic assemblages gives insights into nucleosynthesis and grain condensation in late-stage carbon-rich stars.