Isotopic Studies of Presolar SiC and Oxide Grains as Probes of Stellar Nucleosynthesis and the Chemical Evolution of the Galaxy

This thesis reports the isotopic compositions of different elements in a large number of presolar SiC and oxide grains in order to further increase our knowledge of nucleosynthesis in asymptotic giant branch (AGB) stars, the chemical evolution of the galaxy, and the lifetimes of presolar grains in the interstellar medium. With the NanoSIMS instrument, I have measured the C, N, Si, and Ti isotopic compositions of 238 randomly selected presolar SiC grains. Previous studies of Ti isotopes in presolar SiC are limited and suffer from systematic biases due to non-random grain selection. The grains I have measured allow for a broader picture of the Ti isotopic compositions of presolar SiC, and the observed correlations among the Si and Ti isotopic ratios further confirm the imprint of galactic chemical evolution on the grains\u27 parent stars. I have also revisited presolar SiC grains from the LS and LU fractions from the Murchison meteorite. Presolar SiC from these chemical residues have been previously shown to be morphologically distinct from other presolar SiC, often with irregular shapes and smooth surfaces, and larger in size. In fact, I discovered the largest observed presolar grains to date, with mean diameters of up to 43 μm, and successfully measured the grains\u27 Li, C, and Si isotopic compositions. These measurements yielded the first (and to date, only) Li isotopic anomalies observed in presolar grains, with large enrichments in 6Li produced from cosmic ray spallation. With appropriate production rates, the recoil-loss-corrected individual exposure ages of 8 of these SiC grains were calculated, ranging from 40 Myr to about 1 Gyr. These ages represent the first determination of the interstellar residence times of individual presolar grains. In order to efficiently identify and measure presolar grains, as well as look for ultra-rare grains, I developed an automated measurement routine which was then incorporated into the instrument control software of the NanoSIMS. As a first test of this new system, I analyzed spinel (MgAl2O4) grains from the Murray meteorite, and discovered 41 presolar oxides based on their anomalous O compositions. Of these grains, I measured the Mg-Al isotopic systematics of 26 spinel and 3 Al2O3, and found at least 3 grains with isotopic compositions that are difficult to explain with current models of nucleosynthesis in AGB stars. During these measurements, a grain with large 17O, 25Mg, and 26Mg anomalies was also discovered; the unusual composition of this oxide grain makes it the current best candidate for a nova origin. In order to constrain the chemical composition of presolar spinel, Auger elemental spectra were also taken, and suggest that these grains have stoichiometric Al/Mg ratios, an observation that is in contrast with earlier ionprobe results