Fully Dynamical General Relativistic SPH: Progress and Challenges

The method of Smoothed Particle Hydrodynamics (SPH) has a lot of appeal for simulating variety of catastrophic astrophysical scenarios, such as mergers of compact objects, tidal disruptions, etc. SPH naturally refines dense areas and avoids vacuum without need for a “density floor”. With the recent detections of binary neutron star mergers GW170817 and GW190425 by the LIGO/Virgo collaboration, there is an increasing demand for better understanding of such mergers. Of particlar interest is the amount and composition of the neutron-rich matter ejected during the merger, as it harbors robust rapid neutron capture nucleosynthsis. Several previous works explored SPH with various degrees of approximations to general relativity, and a novel recent study proposed a fully dynamical relativistic SPH (Rosswog & Diener 2021). In this study, we explore another SPH approach to fully general relativistic dynamical geometry with a new code SPaRTA. It uses the Generalized Harmonic formulation of Einstein’s equations for evolving the dynamical metric. For the hydro, SPaRTA builds upon the fixed-metric SPH approach of Tejeda et al. (2017), generalizing it to time-dependent curvilinear geometries. We will discuss progress and challenges, and present several test problems.