Inclusion of isomers/astromers in s-process nucleosynthesis: the pivotal case of 85Kr

J. Tannous, G. W. Misch, M. Mumpower, B. S. Meyer, F. X. Timmes, A. J. Couture, C. L. Fryer

Published ApJ 986 107 (2025)

85-Kr is an s-process branch point nucleus: the competition between neutron capture and β decay influences the production of nearby nuclides. Its evolution is complicated by the existence of a long-lived isomer that β-decays on a much shorter timescale than the ground state. We calculate the effective thermal transition rates between the ground and isomeric states along with estimates of their thermal β-decay rates in a suite of nucleosynthesis computations to assess the consequences of this isomer on isotopic abundances. A central finding in this study is that the s-process flow with the isomer included under astrophysical conditions relevant for the s-process creates a shift in abundance yield of the important radioactivity 87-Rb by a factor of ∼2.4 relative to the case when the 85-Kr is taken to be internally equilibrated when neutron exposures approach the main branch limit. On the other hand, the 88-Sr/86-Sr ratio is little affected because the bulk of the flow through 85-Kr is still via beta decay, whether the isomer is taken into account or not.