# Composition Effects on Kilonova Spectra and Light Curves: I

## TL;DR

This paper investigates how variations in lanthanide and actinide element concentrations in neutron star merger ejecta influence kilonova spectra and light curves, highlighting potential observational probes for composition analysis.

## Contribution

It provides a detailed analysis of how individual element opacities affect kilonova emission, emphasizing the unique roles of Nd and U in spectral features.

## Key findings

- Lanthanide opacities significantly alter kilonova light curves and spectra.
- Nd abundance can be constrained through optical and infrared observations.
- Uranium opacity impacts early-time emission in the 0.2-1 micron range.

## Abstract

The merger of neutron star binaries is believed to eject a wide range of heavy elements into the universe. By observing the emission from this ejecta, scientists can probe the ejecta properties (mass, velocity and composition distributions). The emission (a.k.a. kilonova) is powered by the radioactive decay of the heavy isotopes produced in the merger and this emission is reprocessed by atomic opacities to optical and infra-red wavelengths. Understanding the ejecta properties requires calculating the dependence of this emission on these opacities. The strong lines in the optical and infra-red in lanthanide opacities have been shown to significantly alter the light-curves and spectra in these wavelength bands, arguing that the emission in these wavelengths can probe the composition of this ejecta. Here we study variations in the kilonova emission by varying individual lanthanide (and the actinide uranium) concentrations in the ejecta. The broad forest of lanthanide lines makes it difficult to determine the exact fraction of individual lanthanides. Nd is an exception. Its opacities above 1 micron are higher than other lanthanides and observations of kilonovae can potentially probe increased abundances of Nd. Similarly, at early times when the ejecta is still hot (first day), the U opacity is strong in the 0.2-1 micron wavelength range and kilonova observations may also be able to constrain these abundances.