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Context. The 1.8 MeVγ-rays corresponding to the decay of the radioactive isotope²⁶Al (with a half-life of 0.72 Myr ) have been observed by the SPI detector on the INTEGRAL spacecraft and extensively used as a tracer of star formation and current nucleosynthetic activity in the Milky Way Galaxy. Further information is encoded in the observation related to the higher²⁶Al content found in regions of the Galaxy with the highest line-of-sight (LoS) velocity relative to an observer located in the Solar System. However, this feature remains unexplained. Aims. We ran a cosmological “zoom-in” chemodynamical simulation of a Milky Way-type galaxy, including the production and decays of radioactive nuclei in a fully self-consistent way. We then analyzed the results to follow the evolution of²⁶Al throughout the lifetime of the simulated galaxy to provide a new method for interpreting the²⁶Al observations. Methods. We included the massive star sources of²⁶Al in the Galaxy and its radioactive decay into a state-of-the-art galactic chemical evolution model, coupled with cosmological growth and hydrodynamics. This approach allowed us to follow the spatial and temporal evolution of the²⁶Al content in the simulated galaxy. Results. Our results are in agreement with the observations with respect to the fact that gas particles in the simulation with relatively higher²⁶Al content also have the highest LoS velocities. On the other hand, gas particles with relatively lower²⁶Al content (i.e., not bright enough to be observed) generally display the lowest LoS velocities. However, this result is not conclusive because the overall rotational velocity of our simulated galaxy is higher than that observed for cold CO gas in the Milky Way Galaxy. Furthermore, we found no significant correlation between gas temperature, rotational velocity, and²⁶Al content at any given radius. We also found the presence of transient²⁶Al-rich spots at low LoS velocities and we show that one such spot had been captured by the INTEGRAL/SPI data. Based on our model, we present a prediction for the detection of 1.8 MeVγ-rays by the future COSI mission. We find that according to our model, the new instrument will be able to observe similar²⁶Al-emission patterns to those seen by INTEGRAL/SPI. 

Meteoritic analysis demonstrates that radioactive nuclei heavier than iron were present in the early Solar System. Among them, 129I and 247Cm both have a rapid neutron-capture process (r process) origin and decay on the same timescale (≃ 15.6 Myr). We show that the 129I/247Cm abundance ratio in the early Solar System (438±184) is immune to galactic evolution uncertainties and represents the first direct observational constraint for the properties of the last r-process event that polluted the pre-solar nebula. We investigate the physical conditions of this event using nucleosynthesis calculations and demonstrate that moderately neutron-rich ejecta can produce the observed ratio. We conclude that a dominant contribution by exceedingly neutron-rich ejecta is highly disfavoured. 

Abstract The 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. 

Abstract Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.