Impact of Ejecta Temperature and Mass on the Strength of Heavy Element Signatures in Kilonovae

A kilonova, the electromagnetic emission produced by compact binary mergers, is formed through a delicate interplay of physical processes, involving r-process nucleosynthesis and interactions between heavy elements and photons through radiative transfer. This complexity makes it difficult to achieve a comprehensive understanding of kilonova spectra. In this study, we aim to enhance our understanding and establish connections between physical parameters and observables through radiative-transfer simulations. Specifically, we investigate how ejecta temperature and element mass influence the resulting kilonova spectrum. For each species, the strength of its line features depends on these parameters, leading to the formation of a distinct region in the parameter space, dubbed the Prominent Signature Region (PSR), where the line signature of that species is notably evident in the kilonova spectrum. We explore the origin and applications of PSRs. Among explored r-process elements (31$\leq$Z$\leq$92), we find that four species -- Sr$_{\rm II}$, Y$_{\rm II}$, Ba$_{\rm II}$, and Ce$_{\rm II}$ -- exhibit large and strong PSRs, suggesting their significant contributions to kilonova spectra at specific wavelengths. In addition, we discuss potential challenges and future perspectives in observable heavy elements and their masses in the context of PSRs.
Comment: 13 pages, 10 figures, 1 table, ApJ accepted