Interstellar SiC grains in meteorites

Abstract

It is now well accepted that interstellar grains can be extracted from acid residues of primitive meteorites. One of the easiest materials to recognise is silicon carbide because its overall average ¹²C/¹³C composition is about a factor of two lower than the solar system. The silicon carbide, however, appears to have contributions from three distinct types of grains, two of which are presolar but the third could have originated in the solar system. The components are different in grain size, morphology and in terms of their carbon and silicon isotopes. Nitrogen and noble gases, which occur as trace elements, have abundances and isotopic composition that also help to distinguish the different SiC contributions. A popular idea is that presolar SiC originates in AGB stars but it is not clear at present whether a small number or many stellar sources have been involved.