Interstellar grains in primitive meteorites: diamond, silicon carbide, and graphite

Abstract

Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000-fold for elements such as C and N. Most grains isolated thus far are stable only under highly reduced conditions (C/O>1), and apparently are "stardust' formed in stellar atmospheres. Microdiamonds, of medium size ~10 Å, are most abundant (~400-1800 ppm) but least understood. Silicon carbide shows the signature of the s-process and apparently comes mainly from red giant carbon (AGB) stars. Graphite spherules contain highly anomalous C and noble gases, as well as large amounts of fossil 26Mg from the decay of extinct 26Al. They seem to come from at least three sources, probably AGB stars, novae, and Wolf-Rayet stars. -from Authors