Abstract
AbstractThe cosmic evolution of the chemical elements from the Big Bang to the present time is driven by nuclear fusion reactions inside stars and stellar explosions. A cycle of matter recurrently re-processes metal-enriched stellar ejecta into the next generation of stars. The study of cosmic nucleosynthesis and this matter cycle requires the understanding of the physics of nuclear reactions, of the conditions at which the nuclear reactions are activated inside the stars and stellar explosions, of the stellar ejection mechanisms through winds and explosions, and of the transport of the ejecta towards the next cycle, from hot plasma to cold, star-forming gas. Due to the long timescales of stellar evolution, and because of the infrequent occurrence of stellar explosions, observational studies are challenging, as they have biases in time and space as well as different sensitivities related to the various astronomical methods. Here, we describe in detail the astrophysical and nuclear-physical processes involved in creating two radioactive isotopes useful in such studies,$^{26}\mathrm{Al}$and$^{60}\mathrm{Fe}$. Due to their radioactive lifetime of the order of a million years, these isotopes are suitable to characterise simultaneously the processes of nuclear fusion reactions and of interstellar transport. We describe and discuss the nuclear reactions involved in the production and destruction of$^{26}\mathrm{Al}$and$^{60}\mathrm{Fe}$, the key characteristics of the stellar sites of their nucleosynthesis and their interstellar journey after ejection from the nucleosynthesis sites. This allows us to connect the theoretical astrophysical aspects to the variety of astronomical messengers presented here, from stardust and cosmic-ray composition measurements, through observation of$\gamma$rays produced by radioactivity, to material deposited in deep-sea ocean crusts and to the inferred composition of the first solids that have formed in the Solar System. We show that considering measurements of the isotopic ratio of$^{26}\mathrm{Al}$to$^{60}\mathrm{Fe}$eliminate some of the unknowns when interpreting astronomical results, and discuss the lessons learned from these two isotopes on cosmic chemical evolution. This review paper has emerged from an ISSI-BJ Team project in 2017–2019, bringing together nuclear physicists, astronomers, and astrophysicists in this inter-disciplinary discussion.
Highlights
Understanding the cosmic evolution of the composition of matter from the Big Bang until the present time requires tracing the ensemble of atomic nuclei through their nuclear transformations elements, including C to U, that enables biological life
We have to understand the nucleosynthesis processes themselves, within stars and stellar explosions, that modify the nuclear composition; the nuclear reactions here mostly occur in low-probability tails at energies of tens of keV, which in many cases is far from what we can study by experiments in terrestrial laboratories, so that often sophisticated extrapolations are required
This review focuses on discussion of these two specific isotopes, in relation to the nuclear and astrophysical processes involved in the cycle of matter that drives cosmic chemical evolution
Summary
Elements, including C to U, that enables biological life. This process is called ‘chemical evolution’.a In this review, we will disentangle the processes involved by picking specific nuclei as examples, and tracing their origins and cosmic journey to us. The detection of characteristic 26Al decay γ rays (Mahoney et al 1982) was the first direct proof that nucleosynthesis must be ongoing within the current Galaxy, because this isotope has a characteristic decay half-life of 0.72 Myr, much shorter than the age of the Galaxy, more than 10 Gyr. 26Al, and, 60Fe (with a half-life of 2.62 Myr), both probe recent nucleosynthesis and ejecta transport They have been measured in γ rays from interstellar space, have been found in terrestrial deposits, and have even been inferred to exist in specific abundance in the first solids that formed in the Solar System 4.6 Gyr ago. Our conclusions (Sections 5) summarise the nuclear physics, astrophysics, astronomical, and methodological issues, and the lessons learned as well as the open questions from the study of 26Al and 60Fe in the context of cosmic chemical evolution
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