Radiation hydrodynamics modeling of kilonovae with SNEC

TL;DR

This paper presents a new method combining numerical relativity and radiation-hydrodynamics to simulate kilonova light curves, analyzing various physical effects and comparing results with observations, highlighting areas for further model improvements.

Contribution

The authors develop a comprehensive approach to model kilonova light curves using SNEC and relativistic simulations, including detailed physics and validation against semi-analytic models.

Findings

Hydrodynamic effects are generally negligible in kilonova evolution.

Homologous expansion is a valid approximation in most cases.

Current models do not match the observed kilonova GW170817, indicating model limitations.

Abstract

We develop a method to compute synthetic kilonova light curves that combines numerical relativity simulations of neutron star mergers and the $\texttt{SNEC}$ radiation-hydrodynamics code. We describe our implementation of initial and boundary conditions, r-process heating, and opacities for kilonova simulations. We validate our approach by carefully checking that energy conservation is satisfied and by comparing the $\texttt{SNEC}$ results with those of two semi-analytic light curve models. We apply our code to the calculation of color light curves for three binaries having different mass ratios (equal and unequal mass) and different merger outcome (short-lived and long-lived remnants). We study the sensitivity of our results to hydrodynamic effects, nuclear physics uncertainties in the heating rates, and duration of the merger simulations. We find that hydrodynamics effects are typically negligible and that homologous expansion is a good approximation in most cases. However, pressure forces can amplify the impact of uncertainties in the radioactive heating rates. We also study the impact of shocks possibly launched into the outflows by a relativistic jet. None of our models match AT2017gfo, the kilonova in GW170817. This points to possible deficiencies in our merger simulations and kilonova models which neglect non-LTE effects and possible additional energy injection from the merger remnant, and to the need to go beyond the assumption of spherical symmetry adopted in this work.