A Systematic Multi-Element Isotopic Study of Individual Presolar Graphite Grains

Abstract

Presolar grains are stardust that condensed before the formation of our solar system. This dissertation is a study of a large population of the graphitic presolar grains from two primitive meteorites: Murchison and Orgueil. We used a novel mass spectrometry technique, called Resonance Ionization Mass Spectrometry, to analyze isotopes of multiple heavy trace elements in individual grains, which enabled detection of stellar nucleosynthetic signatures originating in their subgrains. We found evidence of the common s-process nucleosynthesis and a number of rare nucleosynthesis processes, including excesses in p-nuclides of Sr (⁸⁴Sr) and Mo (^92,94Mo), as well as in r-process isotopes of 100Mo and ⁹⁶Zr. Using coordinated light and heavy element isotopic data and astrophysical models, we establish new stellar sites for ⁹⁶Zr production: born-again Asymptotic Giant Branch stars and rapidly accrediting white dwarfs. Furthermore, the ⁸⁴Sr excesses provide direct physical confirmation of the g-process nucleosynthesis in core-collapse supernovae. Our calculations also demonstrate heterogeneous mixing between supernova ejecta layers. We analyzed grains across both light and heavy density fractions of presolar graphite. The study corroborates previous conclusions that high density presolar graphite grains originate from a wide range of stellar sources. In contrast, low density graphites display isotopic evidence of origins in only corecollapse supernovae. Overall, we not only identify the stellar sources of around one hundred stardust grains but also provide astrophysical constraints to various nucleosynthetic reactions. This work fills a large gap in the knowledge of heavy element isotopic abundances in presolar graphite grains.