Deciphering Galactic Chemical Evolution: Iron and Nickel Isotopes and Cosmic Ray Exposure Ages of Presolar Silicon Carbide Grains

Abstract

The universe we live in originated in the Big Bang, which also produced the most abundant elements that are around today, namely hydrogen and helium. However, looking around on Earth today, many more elements exist around and inside of us. For example, humans are a carbon-based life form surviving by breathing nitrogen and oxygen and eating complex molecules. Elements heavier than lithium were not formed in the Big Bang. Most of them were synthesized in evolved stars, except for lithium, beryllium and boron, which were mostly formed by spallation reactions of cosmic rays with interstellar matter. This means that the composition that we find in the Solar System today represents the galactic chemical evolution of the elements and isotopes as it was 4.567 billion years ago. This composition can be studied by analyzing meteorites, which are rocks from space that regularly fall to Earth. Some of these meteorites have not significantly been altered throughout the history of the Solar System and therefore preserved a record of the original composition of the Solar System. In addition, these meteorites contain tiny, micrometer-sized, dust grains that did not form in the Solar System itself. These dust grains - presolar grains - formed in the outflow of dying stars, therefore allowing us to study how a specific star forms elements. In this work we are looking at two aspects of presolar grain research. We first describe the newly built Chicago Instrument for Laser Ionization (CHILI) and compare it to the previous generation instruments at Argonne National Laboratory. We used CHILI to study the iron and nickel isotopic composition of presolar silicon carbide grains. While the neutron-rich isotopes Fe58 and Ni64 are mostly influenced by the parent star from which the grains formed, the neutron-poor isotopes are minimally altered and represent the composition of what went into the star in the first place. Therefore, iron and nickel isotopes are valuable in tracing nucleosynthesis in the parent star as well as to study galactic chemical evolution. Our study finds a good agreement with stellar models in terms of isotope anomalies in the neutron-rich isotopes and a good agreement with galactic chemical evolution models in the neutron-poor isotopes. It however remains a puzzle, how and why the galactic chemical evolution dominated isotopes in presolar silicon carbide grains show such a broad variety in isotopic composition compared to the Solar System. The second part of this work focuses on the age of presolar silicon carbide grains. While these grains were transported through the interstellar medium, they were irradiated with galactic cosmic rays that induced nuclear reactions and yielded the production of so-called cosmogenic nuclides. These nuclides can be used in order to determine the time a presolar grain was exposed to the galactic cosmic ray flux. Here, we present a new model for cosmogenic production rates and discuss in detail the uncertainties that go into the model. We find that Ne21 is the most reliable cosmogenic nuclide to determine a cosmic ray exposure age of a presolar silicon carbide grain. Most presolar silicon carbide grains have ages between 10 and 200 Ma, with a clear peak in the distribution at around 20 Ma. Our new iron and nickel isotopic measurements give tighter constraints on current galactic chemical evolution models. These measurements represent some of the first analyses with the Chicago Instrument for Laser Ionization. With this instrument, many more interesting galactic chemical evolution-dominated elements will be measured in the future, e.g., chromium and titanium, since these elements have only minimal contributions from the grain's parent star. In addition, correlated cosmogenic helium, lithium, and neon studies of individual presolar silicon carbide grains are currently in progress. Such studies will help to better understand current limitations of the cosmic-ray-induced production rates and recoil loss models.