Exploring the diversity of kilonovae with 3D radiative transfer I. The polar direction

TL;DR

This study uses 3D radiative transfer simulations of binary neutron star mergers to analyze kilonova spectra, focusing on polar emissions and comparing results with AT2017gfo observations.

Contribution

It introduces detailed 3D models with new atomic data, highlighting the impact of dynamical ejecta on early kilonova spectral features.

Findings

Simulated spectra show strong Sr II, La III, Gd III, Ce III features similar to AT2017gfo.

Lowest lanthanide fraction models reveal Y II features.

Spectral properties are less sensitive to merger configuration than to ejecta composition.

Abstract

We present 3D kilonova radiative transfer simulations for a series of binary neutron star merger models. The masses of the neutron stars are varied as well as the total mass of the system and two different equations of state were used (SFHO and DD2), producing a range in dynamical ejecta masses and elemental abundance patterns. In this paper, we focus on the bolometric light curves and spectra in the polar direction for comparison with observations of the kilonova AT2017gfo. We calculate line-by-line opacities and include new calibrated lanthanide atomic data. All of the simulated spectra show strong features from Sr II, La III, Gd III and Ce III, which appear to correspond to features identified in AT2017gfo, although the simulated features are generally more blueshifted. The models with the lowest lanthanide fraction in the polar direction also show a Y II feature. Ce III, Ce II, Nd III and Nd II play an important role in shaping the spectral continuum. While the bolometric luminosities in the polar direction vary with the ejecta mass of each model, we find only little sensitivity of the spectral properties to the merger configuration. Our study demonstrates that dynamical ejecta alone can reproduce (although at earlier times) many spectral properties of AT2017gfo, suggesting dynamical ejecta may have a strong impact on the early spectral evolution. However, future simulations are needed to also elucidate the role of other ejecta components for shaping the kilonova spectrum.