Revisiting Near-Infrared Features of Kilonovae: The Importance of Gadolinium

TL;DR

This study identifies Gadolinium (Gd III) as a key element influencing near-infrared spectral features of kilonovae, enhancing understanding of heavy element synthesis in neutron star mergers.

Contribution

It systematically searches for near-infrared transitions of heavy elements, highlighting Gd III as a significant contributor to kilonova spectra, previously unrecognized in this context.

Findings

Gd III shows strong infrared transitions due to its atomic structure.

Gd III absorption lines are observed in a peculiar star, supporting their presence in kilonovae.

Radiative transfer simulations confirm Gd III's impact on near-infrared features.

Abstract

The observation of the kilonova AT2017gfo and investigations of its light curves and spectra confirmed that neutron star mergers are sites of r-process nucleosynthesis. However, the identification of elements responsible for the spectral features is still challenging, particularly at the near-infrared wavelengths. In this study, we systematically searched for all possible near-infrared transitions of heavy elements using experimentally calibrated energy levels. Our analysis reveals that most candidate elements with strong absorption lines are lanthanides (Z=57-71) and actinides (Z=89-103). This is due to their complex structures leading to many low-lying energy levels, which results in strong transitions in the near-infrared range. Domoto et al. (2022) have shown that La III and Ce III can explain the absorption features at $\lambda\sim$ 12,000 - 15,000 A. While our results confirm that these two elements show strong infrared features, we additionally identify Gd III as the next most promising species. Due to its unique atomic structure involving the half-filled 4f and the outer 5d orbitals, Gd III has one of the lowest-lying energy levels, between which relatively strong transitions occur. We also find absorption lines caused by Gd III in the near-infrared spectrum of a chemically peculiar star HR 465, which supports their emergence in kilonova spectra. By performing radiative transfer simulations, we confirm that Gd III lines affect the feature at $\sim$ 12,000 A previously attributed to La III. Future space-based time-series observations of kilonova spectra will allow the identification of Gd III lines.