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

Abstract

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 ($\dot{\epsilon }_{\rm nuc}$) of ejecta from NSMs, taking into account β− decay through isomers. We find that for ejecta with electron fraction (Ye) of 0.2–0.3, $\dot{\epsilon }_{\rm nuc}$ is appreciably changed from those without isomers, due to the large change in timing of β− decay through the isomeric states (ISs) of ${}^{123,125,127} \rm Sn$ and ${}^{128} \rm Sb$. In particular, the effects of the isomers on $\dot{\epsilon }_{\rm nuc}$ are prominent for ejecta of Ye ∼ 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 IS is important, the isomers of ${}^{129} \rm Sb$ and ${}^{129, 131} \rm Te$ also cause additional and appreciable change in $\dot{\epsilon }_{\rm nuc}$. 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.