Inherited material from the protosolar cloud: composition and origin

When the most primitive types of meteorite, the chondrites, formed, all of them incorporated small amounts of at least two types of unprocessed pre–solar material: interstellar organic matter and circumstellar grains. The organic matter formed in the protosolar molecular cloud. To date, six types of circumstellar grain have been identified and their isotopic compositions show that they formed around red giant/asymptotic giant branch (RGB/AGB) stars, supernovae and, possibly, novae and Wolf–Rayet stars. The range of isotopic compositions exhibited by the pre–solar grains requires a minimum of 35–40 stellar sources, suggesting that the circumstellar grains are a fairly representative sample of circumstellar dust production in the Galaxy. Comparison of the observed proportions of circumstellar grains with astronomical estimates reveals both striking similarities and differences. The differences probably reflect our ignorance of circumstellar dust formation rather than differential destruction in the interstellar medium or the Solar System. Silicates are predicted to be one of the dominant types of circumstellar grain, but they have yet to be isolated from meteorites. However, interstellar silicates may have been identified in interplanetary dust particles. The pre–solar materials demonstrate that some matter was never strongly heated during Solar System formation. There is no evidence for one dominant source of circumstellar grains, as might have occurred if Solar System formation was triggered by a supernova/AGB–generated shock wave or if refractory supernova condensates produced the widespread mass–independent oxygen isotopic variations found in Solar System materials.