STELLAR NUCLEOSYNTHESIS AND THE ISOTOPIC COMPOSITION OF PRESOLAR GRAINS FROM PRIMITIVE METEORITES

Primitive meteorites contain grains of stardust that originated from stellar outflows and supernova ejecta prior to the formation of the Solar System. The study of these grains in the laboratory provide new information on stellar evolution, nucleosynthesis, mixing in supernovae, galactic evolution, and the age of the galaxy. Grains whose isotopically anomalous compositions indicate a stellar origin include diamond, silicon carbide, graphite, corundum, and silicon nitride. Most silicon carbide and corundum come from red giant and asymptotic giant branch stars (low-mass stars at the end of their evolution), and carry the isotopic signatures of H burning in the core and later of H and He burning in thin shells. Diamond carries a supernova isotopic signature in its Xe, and low-density graphite and silicon nitride, as well as a subgroup of silicon carbide, show evidence for a supernova origin in the form of extinct ⁴⁴Ti and large ²⁸Si excesses.