Radioactive Decay of Specific Heavy Elements as an Energy Source for Late-Time Kilonovae and Potential JWST Observations

Revealing the temporal evolution of individual heavy elements synthesized in the merger ejecta from binary neutron star mergers not only improves our understanding of the origin of heavy elements beyond iron but also clarifies the energy sources of kilonovae. In this work, we present a comprehensive analysis of the temporal evolution of the energy fraction of each nuclide based on the $r$-process nucleosynthesis simulations. The heavy elements dominating the kilonova emission within $\sim 100$~days are identified, including ${}^{127}$Sb, ${}^{128}$Sb, ${}^{129}$Sb, ${}^{130}$Sb, ${}^{129}$Te, ${}^{132}$I, ${}^{222}$Rn, ${}^{223}$Ra, ${}^{224}$Ra, and ${}^{225}$Ac. It is found that the late-time kilonova light curve ($t\gtrsim 20$~days) is highly sensitive to the presence of the heavy element ${}^{225}$Ac (with a half-life of 10.0~days). Our analysis shows that the James Webb Space Telescope (JWST), with its high sensitivity in the near-infrared band, is a powerful instrument for the identification of these specific heavy elements.