The impact of isomers on a kilonova associated with neutron star mergers

Abstract

We investigate the significance of isomers on a kilonova associated with neutron star mergers (NSMs) for the first time. We calculate the evolution of abundances and energy generation rates (${\dot{ϵ}}_{\mathrm{nuc}}$) of ejecta from NSMs, taking into account ${\beta }^{-}$ decay through isomers. We find that for ejecta with electron fraction ($Y_e$) of $0.2-0.3$, ${\dot{ϵ}}_{\mathrm{nuc}}$ is appreciably changed from those without isomers, due to the large change in timing of ${\beta }^{-}$ decay through the isomeric states of ${}^{123,125,127}\mathrm{Sn}$ and ${}^{128}\mathrm{Sb}$. In particular, the effects of the isomers on ${\dot{ϵ}}_{\mathrm{nuc}}$ are prominent for ejecta of $Y_e\sim 0.25$, which could emit a fraction of an early, blue component of a kilonova observed in GW170817. When the excitation from a ground state to its isomeric state is important, the isomers of ${}^{129}\mathrm{Sb}$ and ${}^{129,131}\mathrm{Te}$ also cause additional and appreciable change in ${\dot{ϵ}}_{\mathrm{nuc}}$. Furthermore, we demonstrate that larger amounts of lanthanide-free ejecta result in a better fit of an observed light curve of the kilonova in GW170817 if the isomers are taken into account.

Comments: 6 pages, 4 figures. Accepted for publication in MNRAS