Astromers: Nuclear Isomers in Astrophysics

TL;DR

This paper introduces a method to compute transition rates between nuclear ground states and long-lived isomers, establishing criteria for when to treat them as separate species in astrophysical models, impacting nucleosynthesis calculations.

Contribution

The authors develop a novel approach to determine transition rates and criteria for nuclear thermalization, enabling accurate treatment of isomers as separate species in astrophysical environments.

Findings

Established criteria for nuclear thermalization temperature.

Demonstrated the importance of treating isomers separately below thermalization.

Performed sensitivity studies on key astrophysical isomers.

Abstract

We develop a method to compute thermally-mediated transition rates between the ground state and long-lived isomers in nuclei. We also establish criteria delimiting a thermalization temperature above which a nucleus may be considered a single species and below which it must be treated as two separate species: a ground state species, and an astrophysical isomer ("astromer") species. Below the thermalization temperature, the destruction rates dominate the internal transition rates between the ground state and the isomer. If the destruction rates also differ greatly from one another, the nuclear levels fall out of or fail to reach thermal equilibrium. Without thermal equilibrium, there may not be a safe assumption about the distribution of occupation probability among the nuclear levels when computing nuclear reaction rates. In these conditions, the isomer has astrophysical consequences and should be treated a separate astromer species which evolves separately from the ground state in a nucleosynthesis network. We apply our transition rate methods and perform sensitivity studies on a few well-known astromers. We also study transitions in several other isomers of likely astrophysical interest.