Presolar graphite grains and their stellar origins: A review

Presolar grains are sub-micron to micron-sized stardust grains that are found in very primitive solar system materials. Laboratory measurements of presolar grains allow cosmochemists to understand the chemical and physical conditions in circumstellar environments, where the grains condensed. They provide important constraints for models of stellar nucleosynthesis and Galactic chemical evolution. In this paper, we review our current knowledge of the stellar sources of presolar graphite grains. This presolar grain species is not as abundant or widely studied as silicon carbide grains but is already known to have a variety of interesting stellar sources. Isotopic and microstructural measurements of more than 2000 graphite grains indicate that ∼ 26% originate in Type II supernovae, ∼ 48% in low-metallicity asymptotic giant branch (AGB) stars, and less than 1% in post-AGB stars and J-type stars. The remaining 25% of the grains have an ambiguous origin. We discuss the available evidence for the various stellar sources and outline the problems that are encountered while comparing grain data with astrophysical models.