Tellurium emission line in kilonova AT 2017gfo

TL;DR

This paper identifies a tellurium emission line in the late-time spectra of kilonova AT 2017gfo, providing insights into the element composition and ejecta mass through spectral modeling.

Contribution

It is the first to attribute the 2.1 μm emission line to Te III in kilonova spectra, linking spectral features to r-process element abundance and ejecta mass.

Findings

Detection of a 2.1 μm emission line attributed to Te III.

Estimated tellurium mass of ~10^{-3} solar masses.

Consistent ejecta mass with light curve estimates.

Abstract

The late-time spectra of the kilonova AT 2017gfo associated with GW170817 exhibit a strong emission line feature at $2.1\,{\rm \mu m}$. The line structure develops with time and there is no apparent blue-shifted absorption feature in the spectra, suggesting that this emission line feature is produced by electron collision excitation. We attribute the emission line to a fine structure line of Tellurium (Te) III, which is one of the most abundant elements in the second r-process peak. By using a synthetic spectral modeling including fine structure emission lines with the solar r-process abundance pattern beyond the first r-process peak, i.e., atomic mass numbers $A\gtrsim 88$, we demonstrate that [Te III] $2.10\,\rm \mu m$ is indeed expected to be the strongest emission line in the near infrared region. We estimate that the required mass of Te III is $\sim 10^{-3}M_{\odot}$, corresponding to the merger ejecta of $0.05M_{\odot}$, which is in agreement with the mass estimated from the kilonova light curve.