The study of radioisotopes in presolar dust grains

Presolar grains in primitive meteorites are characterized by large isotopic anomalies in essentially all the analyzed elements. Among these anomalies are excesses in the daughter isotopes of short-lived radioisotopes. These excesses indicate the incorporation of short-lived now extinct isotopes into the grains at the time of their formation. So far laboratory studies of presolar grains have established evidence for the initial presence of the isotopes V-49, Na-22, Ti-44, Ca-41, Tc-99, and Al-26. Evidence for these isotopes have been found in presolar silicon carbide, graphite, and oxide grains from supernovae (V-49, Na-22, Ti-44, Ca-41, and Al-26) and AGB stars (Ca-41, Tc-99, and Al-26). We analyzed presolar hibonite (CaAl12O19) grains, whose oxygen isotopic ratios indicate an origin in AGB stars, for their Al-Mg and Ca-K isotopic systems, determined the initial Al-26/Al-27 and Ca-41/Ca-40 ratios, and compared them with theoretical models of the production of these isotopes in AGB stars. Whereas the abundance of Ca-41, which is produced by neutron capture, agrees with the models, the abundance of Al-26, which is produced by proton capture, greatly exceeds theoretical predictions. The production of this radioisotope requires a process in addition to H shell burning, cool bottom processing. (c) 2006 Elsevier B.V. All rights reserved.