Simulating Neutron Star Mergers with Fully Dynamical General Relativistic SPH

Smoothed Particle Hydrodynamics (SPH) is a meshfree Lagrangian simulation approach that has a lot of appeal for simulating catastrophic astrophysical scenarios such as mergers of compact objects, tidal disruptions, supernova explosions, etc. It provides excellent conservation properties, natural adaptive refinement in dense areas, and the treatment of vacuum without the need for a “density floor”. Recent groundbreaking detection of binary neutron star mergers GW170817 and GW190425 by gravitational-wave interferometers provides an increasing demand to better understand the mechanics of merging neutron stars and especially their ejecta, which appears to be the best candidates for the primary sites of the robust rapid neutron capture (r-process) nucleosynthesis. Here we demonstrate capability of using SPH in curved dynamic spacetime, with a new code SPaRTA (= Smoothed Particles for Relativistic Theoretical Astrophysics). It uses the Generalized Harmonic formulation of Einstein’s equations for evolving the dynamical spacetime metric, and a time-dependent extension of the fixed-metric Tejeda-Rosswog GRSPH approach. We discuss progress with simulations of binary neutron star mergers. SPaRTA is a parallel distributed multiphysics code. It uses simple block domain decomposition for the grid on which the metric is evolved, and a dependent domain partitioning with a binary kd-tree to handle the particles affinity.