Long-term simulations of dynamical ejecta: Homologous expansion and kilonova properties

Accurate numerical-relativity simulations are essential to study the rich phenomenology of binary neutron star systems. In this work, we focus on the material that is dynamically ejected during the merger process and on the kilonova transient it produces. Typically, radiative transfer simulations of kilonova light curves from ejecta make the assumption of homologous expansion, but this condition might not always be met at the end of usually very short numerical-relativity simulations. In this article, we adjust the infrastructure of the bam code to enable longer simulations of the dynamical ejecta with the aim of investigating when the condition of homologous expansion is satisfied. In fact, we observe that the deviations from a perfect homologous expansion are about 30% at roughly 100 ms after the merger. While the calculation of the kilonova light curves is affected by the resolution as well as our method of simplifying the ejecta simulation, these deviations from the homologous expansion also bias the results. We determine this influence by extracting the ejecta data for different reference times and use them as input to radiative transfer simulations. Our results show that the light curves for extraction times later than 80 ms after the merger deviate by 0.4 mag and are mostly consistent with numerical noise. Accordingly, deviations from the homologous expansion for the dynamical ejecta component are negligible from ∼80 ms for the purpose of kilonova modeling.