Supernova grains from meteorites

Primitive meteorites contain presolar grains that originated in stellar outflows and supernova ejecta. The isotopic compositions of some of these grains (low-density graphite grains, SiC grains of type X, and silicon nitride) indicate a supernova origin. In particular, the initial presence of Ti-44 (now detected in the form of large Ca-44 excesses) and Si-28 excesses are proof that the carrier grains formed in supernova ejecta. The presence of these two isotopes, which are produced in the inner layers of the star, in carbonaceous grains is evidence for extensive mixing of different supernova layers in the explosion. Other isotopic signatures characteristic of supernova grains are N-15 and O-18 excesses, large inferred Al-26/Al-27 and Ca-41/Ca-40 ratios and excesses in Ca-42, Ca-43, and Ti-49. Mixing of supernova layers can explain most isotopic features but a few problems such as the association of N-15 with Al-26 in grains remain. The fact that all supernova grains identified so far are graphite, SiC and Si3N4 and no oxide grains with supernova signatures have been found remains a puzzle.