Astromers: Astrophysical Isomers

Astrophysical nucleosynthesis simulations require accurate nuclear data input, especially the reaction cross sections and decay rates that connect isotopes in a nucleosynthesis network. In most calculations, the inputs typically come from either ground state properties or a thermal equilibrium population of excited states; however, nuclear isomers (metastable excited states) cause some isotopes to behave differently. Some isomers retain their metastibility in astrophysical nucleosynthesis sites, making them astrophysical isomers, or "astromers". When this occurs, such an astromer can undergo transmutation (reaction or decay) at a rate very different from the ground state, and its relative population may be driven far from thermal equilibrium. As a consequence, neither the ground-state nor thermal-equilibrium nuclear properties apply, and the isotope may need to be included in a nucleosynthesis simulation as multiple species: a ground state, and one or more astromers. This issue has been recognized for over forty years, but it has been carefully studied in only a small handful of isotopes. We are at last entering an era of broad research on the topic of astromers, including identifying which are important, new ways to study them in simulations, and what properties are essential to measure experimentally.