Presolar grains in meteorites: an overview and some implications

An overview is given of the types and properties of grains of presolar orig in that have been identified in primitive meteorites. Presolar diamonds are most abundant (up to > 10/00), but it is not clear, what fraction truly is presolar, and they represent the most enigmatic case. The situation is dif ferent for the other identified presolar grains, which are less abundant by two orders of magnitude or more. The case is most clear-cut for the silico n carbide grains most of which ( > 90%) originate from asymptotic giant bra nch stars, with a small fraction showing the signatures of a supernova orig in. Graphite grains require an origin from a variety of stellar objects, wh ile the primary source of aluminum oxide grains are Red Giants. The silicon nitride grains identified so far show the isotopic signatures of supernova material. The constraints provided by the isotopic composition of heavy trace element s in SIC and diamond are discussed. Those occurring in the "mainstream" SiC grains show the signature of the slow neutron capture process (s-process) occurring in AGE stars, a major source of the heavy elements in the Solar S ystem, while heavy trace elements contained in the diamonds are probably ti ed to rapid neutron capture in exploding supernovae. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.