Dust Condensation in Circumstellar Environments: Insight from Chemical and Microstructural Analyses of Presolar Grains

Abstract

Presolar grains are among the original building blocks of our solar system from which the sun and planets formed. These grains formed in the circumstellar envelopes of evolved stars and in the ejecta of stellar explosions such as supernovae. Analysis of them provides ground-truth to astronomical observations and insights into their stellar origins. Silicates are a major dust component in the gaseous envelopes surrounding O-rich stars and the interstellar medium (ISM), and the analysis of them can provide a wealth of information about the thermodynamic landscape of circumstellar environments and supernovae. Further, such materials offer insight into transport through the interstellar medium, preservation in solar system materials, and any secondary alteration they may have experienced since their formation. This dissertation is broadly focused on studying silicate presolar grains preserved in primitive meteorites to investigate their structures and chemistries, understand how they are affected by secondary alteration, and how they are preserved in different chondrite groups. This work is supported by isotopic measurements using nanoscale secondary-ion mass spectrometry (NanoSIMS), but is focused on structural and chemical characterization using advanced electron microscopy techniques including focused-ion beam scanning-electron microscopy (FIB-SEM) and transmission electron microscopy (TEM). Chapter 1 of this dissertation provides an introduction to molecular clouds, star formation, stellar nucleosynthesis, the injection of matter into the ISM, formation of the solar protoplanetary disk, incorporation of gas and dust from the ISM, and preservation of presolar grains in planetary materials such as meteorites. Chapter 2 describes the analytical methods used to complete this work including NanoSIMS, Auger spectroscopy, FIB-SEM, and TEM. Three subsequent research chapters aim to fill knowledge gaps in presolar-silicate research to date. First, very few presolar silicate grains with origins in supernovae have been analyzed for their detailed structure and chemistry. Chapter 3 is focused on understanding the types of silicates that form in supernova environments, the thermodynamics of their formation, and comparing such data to presolar grains from other stellar origins. Second, presolar silicates are easily destroyed by secondary alteration in the solar nebula and on the parent asteroid and are therefore useful tracers of processing such as aqueous alteration and thermal metamorphism. Chapter 4 describes the alteration histories of presolar silicates in carbonaceous chondrites, particularly how they are preserved in solar system materials, the mechanisms by which they alter, and the extent to which they are affected by alteration processes. Finally, very little detailed structural and chemical characterization has been completed on presolar grains preserved in ordinary chondrites. The ordinary and carbonaceous chondrites are believed to have formed in separate reservoirs in the solar protoplanetary disk and therefore, analyzing presolar grains from both reservoirs could give insight into how the protoplanetary disk was seeded. Chapter 5 is focused on the analysis of presolar silicates in ordinary chondrites and comparing such data to presolar grain populations in carbonaceous chondrites and interplanetary dust particles to better understand if there are differences in presolar grains that were preserved in different chondrite reservoirs. Chapters 3 and 4 were published in Meteoritics & Planetary Science and Chapter 5 was submitted to Meteoritics & Planetary Science.