Recent Studies of Presolar Grains.

Presolar grains are defined as grains which had formed in the circumstellar envelope or supernova ejecta and remain intact during the formation of the solar system. They have been extensively studied during the last 10 years. They exhibit huge isotopic anomalies, confirming their stellar origin. SIMS (Secondary Ion Mass Spectrometry) has proved itself to be a very powerful tool to study presolar grains. Isotopic ratios of more than one elements can be measured if grains are 1 μm or larger in size. Several kinds of stars have been identified as stellar sources of presolar grains. One type is AGB (Asymptotic Giant Branch) stars and another type is supernovae (SNe). Most presolar SiC grains (about 93%) exhibit 12C/13C ratios ranging from 30 to 100 and are called mainstream SiC grains. Their C and N isotopic ratios generally agree with what is expected from the evolution of AGB stars. Silicon isotopic ratios of the grains show excesses in 29Si and 30Si. To explain the Si-isotopic ratios, Galactic chemical evolution or inhomogeniety of the ISM (Interstellar Medium) has to be invoked. Silicon carbide of type × (about 1% of total SiC), low-density graphite, and silicon nitride are most likely to have formed in supernova ejecta. Many grains have 28Si excesses and a few grains show the presence of 44Ti. These features are considered to be a proof of their supernova origin. The comparison with theoretical calculations of nucleosynthesis in supernova suggests that extensive and heterogeneous mixing occurs when supernovae explode.