Properties of Kilonovae from Dynamical and Post-Merger Ejecta of Neutron Star Mergers

TL;DR

This paper investigates the atomic properties of r-process elements in neutron star merger ejecta, revealing how different element compositions influence kilonova brightness and color, with implications for observational detection.

Contribution

It provides atomic structure calculations for key r-process elements and analyzes how their opacities affect kilonova emission properties, especially brightness and color variations.

Findings

Lanthanide elements have higher opacities than lighter elements.

Blue optical emission can be observed from Lanthanide-free ejecta.

Detectable magnitudes for blue emission are within reach of small telescopes.

Abstract

Ejected material from neutron star mergers give rise to electromagnetic emission powered by radioactive decays of r-process nuclei, which is so called kilonova or macronova. While properties of the emission are largely affected by opacities in the ejected material, available atomic data for r-process elements are still limited. We perform atomic structure calculations for r-process elements: Se (Z=34), Ru (Z=44), Te (Z=52), Ba (Z=56), Nd (Z=60), and Er (Z=68). We confirm that the opacities from bound-bound transitions of open f-shell, Lanthanide elements (Nd and Er) are higher than those of the other elements over a wide wavelength range. The opacities of open s-shell (Ba), p-shell (Se and Te), and d-shell (Ru) elements are lower than those of open f-shell elements and their transitions are concentrated in the ultraviolet wavelengths. We show that the optical brightness can be different by >2 mag depending on the element abundances in the ejecta such that post-merger, Lanthanide-free ejecta produce brighter and bluer optical emission. Such blue emission from post-merger ejecta can be observed from the polar directions if the mass of the preceding dynamical ejecta in these regions is small. For the ejecta mass of 0.01 Msun, observed magnitudes of the blue emission will reach 21.0 mag (100 Mpc) and 22.5 mag (200 Mpc) in g and r bands within a few days after the merger, which are detectable with 1m or 2m-class telescopes.