Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006iaujd..11e...9h&link_type=abstract
Pre-Solar Grains as Astrophysical Tools, 26th meeting of the IAU, Joint Discussion 11, 21 August 2006, Prague, Czech Republic, J
Astronomy and Astrophysics
Astrophysics
Scientific paper
The laboratory study of presolar grains has made important contributions to several areas of astronomy and astrophysics. For example, studying isotopic compositions of grains that condensed from the ejecta of dying stars provides "ground truth" for models of stellar nucleosynthesis. Presolar grains are also excellent probes of conditions in the Solar Nebula (accretion disk) during the earliest stages of solar-system formation. Known types of presolar grains have a wide range of thermal and chemical resistance. Aliquots of molecular cloud material heated to different degrees in the Solar Nebula should show differences in abundances of surviving presolar material that reflect the degree of heating (and/or oxidation state). A few chondrites in each of several chemical classes were not heated enough after accretion to affect the abundances of presolar grains. Among these meteorites, the relative abundances of presolar diamond, silicon carbide, and graphite, and the carrier of P1 (= Q) noble gases, which may also be presolar, show a close correlation to the contents of moderately volatile elements in the host meteorites. This is true even though the presolar grains track the thermal history of the fine-grained "matrix" of the meteorites, while the bulk compositions reflect all components of the meteorites. The correlation implies that the same nebular processing was responsible for changing both the abundances of presolar grains and the bulk compositions from those of the original molecular cloud material. The processing apparently occurred before chondrules, the main constituent of chondrites, formed. Episodes of high temperature sufficient to completely evaporate molecular cloud dust are not required to produce the chondrites. In fact, they are precluded because such temperatures would destroy all of the presolar grains. Crystalline forsterite and enstatite may also be tracers of nebular processing. Astronomical observations suggest that interstellar dust is primarily amorphous and that crystalline material appears only after heating in the accretion disk. Laboratory work on these phases in primitive materials is just starting, but should help with the interpretation of the astronomical observations.
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