10:45 AM - 11:05 AM
[MZZ45-01] Nucleosynthesis: The Where and How
★Invited Papers
Keywords:Nucleosynthesis, Isotope anomaly, Meteorites, Solar System
There are about 90 elements that occur naturally on Earth. With the addition of unstable radioactive elements that can only be artificially synthesized in laboratories, the presence of 118 elements has been confirmed. The physical properties of an atom are characterized by the number of protons and neutrons that make up the atomic nucleus, which ultimately control the nature of the element. In the universe, the essential light elements, including 1H, 2H, 3He, and 4He, were first synthesized by the Big Bang, followed by the production of new, heavier atomic nuclei through nuclear fusion reactions in stars using pre-existing atomic nuclei, protons, neutrons, and elementary particles. Main-sequence stars, such as the Sun today, fuse 1H to produce 4He. In the red giant (RG) sequence after the main sequence stage, 12C and 16O are produced by the fusion of 4He. The subsequent stellar evolution and nucleosynthesis depends on the initial mass of the star. Low- to moderate-mass stars evolve to the asymptotic giant branch (AGB) stage, an important source of C, N, and O in galaxies. The AGB stars evolve into white dwarfs, where no nucleosynthesis occurs, while some AGB stars in binary systems end up as Type Ia supernovae, producing 56Fe and other Fe-peak isotopes. In contrast, massive stars in the RG stage are capable of initiating C burning to synthesize 20Ne, followed by sequential reactions of Ne-, O-, and Si-burning that continue until the formation of 56Ni. The core of massive stars then loses pressure and begins to gravitationally collapse, causing the explosion of the outer layers (core-collapse supernova), during which some explosive nucleosynthesis occurs. Meanwhile, nuclides heavier than Fe are synthesized by the s- and r-processes, which occur in various stellar locations as well as in neutron star mergers. On the other hand, additional processes are required to account for some minor nuclides such as Li, Be, B, and p-nuclides.
Historically, stellar nucleosynthesis has been studied using astronomical and theoretical approaches. More recently, innovations in mass spectrometric techniques have allowed the precise determination of isotopic compositions for a variety of elements in meteorites and their constituents. Isotopic analyses of presolar grains have constrained the physical conditions of the stellar environments from which these grains were derived. Isotopic compositions of refractory materials in primitive chondrites, including calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), are clearly distinguishable from bulk meteorites, presumably due to the large contribution of supernova-generated materials. In more detail, subtle differences in isotopic compositions can be found between bulk rocks of carbonaceous chondrites (CC) and non-carbonaceous chondrites (NC) for elements such as Cr, Ti, and Mo. The observed differences are now recognized as isotopic dichotomy of meteorites, indicating the heterogeneous distribution in the solar nebula of materials formed by different stellar environments prior to the birth of the Solar System. In this talk, I will review stellar nucleosynthesis and present recent advances in the cosmochemical application of high-precision isotopic analysis of extraterrestrial materials related to this topic.
Historically, stellar nucleosynthesis has been studied using astronomical and theoretical approaches. More recently, innovations in mass spectrometric techniques have allowed the precise determination of isotopic compositions for a variety of elements in meteorites and their constituents. Isotopic analyses of presolar grains have constrained the physical conditions of the stellar environments from which these grains were derived. Isotopic compositions of refractory materials in primitive chondrites, including calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), are clearly distinguishable from bulk meteorites, presumably due to the large contribution of supernova-generated materials. In more detail, subtle differences in isotopic compositions can be found between bulk rocks of carbonaceous chondrites (CC) and non-carbonaceous chondrites (NC) for elements such as Cr, Ti, and Mo. The observed differences are now recognized as isotopic dichotomy of meteorites, indicating the heterogeneous distribution in the solar nebula of materials formed by different stellar environments prior to the birth of the Solar System. In this talk, I will review stellar nucleosynthesis and present recent advances in the cosmochemical application of high-precision isotopic analysis of extraterrestrial materials related to this topic.