Cosmic Radioactivity and Galactic Chemical Evolution

Abstract

The description of the tempo-spatial evolution of the composition of cosmic gas on galactic scales is called ‘modelling galactic chemical evolution’. It aims to use knowledge about sources of nucleosynthesis and how they change the composition of interstellar gas, following the formation of stars and the ejection of products from nuclear fusion during their evolution and terminating explosions. Sources of nucleosynthesis are diverse: Stars with hydrostatic nuclear burning during their evolution shed parts of the products in planetary nebulae, winds, and core-collapse supernovae. Binary interactions are important and lead to important sources, such as thermonuclear supernovae and kilonovae. Tracing ejecta from sources, with their different frequencies and environments, through the interstellar medium and successive star formation cycles is the goal of model descriptions. A framework that traces gas and stars through star formation, stellar evolution, enriched-gas ejections, and large-scale gas flows is formulated. Beyond illustrating the effects of different assumptions about nucleosynthesis sources and gas recycling, this allows us to interpret the large amount of observational data concerning the isotopic composition of stars, galaxies, and the interstellar medium. A variety of formalisms exist, from analytical through semi-analytical, numerical, or stochastic approaches, gradually making descriptions of compositional evolution of cosmic matter more realistic, teaching us about the astrophysical processes involved in this complex aspect of our universe. Radioactive isotopes add important ingredients to such modelling: The intrinsic clock of the radioactive decay process adds a new aspect to the modelling algorithms that leads to different constraints on the important unknowns of star formation activity and interstellar transports. Several prominent examples illustrate how modelling the abundances of radioactive isotopes and their evolutions has resulted in new lessons; among these are the galaxy-wide distribution of26Al and 60Fe, the radioactive components of cosmic rays, the interpretations of terrestrial deposits of 60Fe and 244Pu, and the radioactive decay daughter isotopes that were found in meteorites and characterise the birth environment of our solar system.