LABORATORY STUDIES OF CIRCUMSTELLAR AND INTERSTELLAR MATERIALS

When the most primitive types of meteorite (chondrites) and interplanetary dust particles (IDPs) formed in the solar protoplanetary disk (solar nebula), all of them incorporated at least two types of presolar material: interstellar organic matter and circumstellar grains. The organic matter probably formed in the protosolar molecular cloud. To date, 10 types of circumstellar grain have been found and their isotopic compositions show that they formed around RGB/AGB stars, supernovae and, possibly, novae and WR stars. It has been proposed that interstellar silicates are also present; in IDPs, but the evidence for this is less compelling. The range of isotopic compositions exhibited by the circumstellar grains require a minimum of 35-40 stellar sources, suggesting that; they are a fairly representative sample of circumstellar dust production in the Galaxy. In general, the relative abundances of circumstellar grains from different sources are similar to what is expected from astronomical estimates provided that the contribution from supernovae is relatively small. The one exception is graphite grains from AGB stars that are highly depleted. Collisional erosion, radiation damage and sputtering are all expected to affect grains in the ISM on relatively short timescales. Yet, for reasons that are unclear, examination of the circumstellar grains has found little evidence for any of these processes. The circumstellar grains also provide information about grain nucleation and growth in stellar winds. For instance, only graphite grains from AGB stars seem to have formed by heterogeneous nucleation. Graphite and SiC grains from AGB stars as large as 10-20 mu m have been found in meteorites, probably requiring regions of high gas density rather than the usual assumption of uniformly expanding outflows. However, few graphite and SiC grains have similar C isotopic compositions, suggesting that they tend to form at different stages of AGB evolution.