Planetary Giant Impacts: Convergence of High-Resolution Simulations using Efficient Spherical Initial Conditions and SWIFT

We perform simulations of giant impacts onto the young Uranus using smoothed particle hydrodynamics (SPH) with over 100 million particles. This 100--1000$\times$ improvement in particle number reveals that simulations with below 10^7 particles fail to converge on even bulk properties like the post-impact rotation period, or on the detailed erosion of the atmosphere. Higher resolutions appear to determine these large-scale results reliably, but even 10^8 particles may not be sufficient to study the detailed composition of the debris -- finding that almost an order of magnitude more rock is ejected beyond the Roche radius than with 10^5 particles. We present two software developments that enable this increase in the feasible number of particles. First, we present an algorithm to place any number of particles in a spherical shell such that they all have an SPH density within 1% of the desired value. Particles in model planets built from these nested shells have a root-mean-squared velocity below 1% of the escape speed, which avoids the need for long precursor simulations to produce relaxed initial conditions. Second, we develop the hydrodynamics code SWIFT for planetary simulations. SWIFT uses task-based parallelism and other modern algorithmic approaches to take full advantage of contemporary supercomputer architectures. Both the particle placement code and SWIFT are publicly released.