Constraints on Stellar Grain Formation from Presolar Graphite in the Murchison Meteorite

We report the results of isotopic, chemical, structural, and crystallographic microanalyses of graphitic spherules (0.3-9 μm) extracted from the Murchison meteorite. The spherules have12C/13C ratios ranging over 3 orders of magnitude (from 0.02 to 80 times solar), clearly establishing their presolar origin as stellar condensates. These and other isotopic constraints point to a variety of stellar types as sources of the carbon, including low-mass asymptotic giant branch (AGB) stars and supernovae. Transmission electron microscopy (TEM) of ultrathin sections of the spherules revealed that many have a composite structure consisting of a core of nanocrystalline carbon surrounded by a mantle of well-graphitized carbon. The nanocrystalline cores are compact masses consisting of randomly oriented graphene sheets, from PAH-sized units up to sheets 3-4 nm in diameter, with little graphitic layering order. These sheets probably condensed as isolated particles that subsequently coalesced to form the cores, after which the surrounding graphitic mantles were added by vapor deposition.

Перевод пока недоступен