BIFROST: simulating compact subsystems in star clusters using a hierarchical fourth-order forward symplectic integrator code

TL;DR

BIFROST is a GPU-accelerated simulation code for star clusters that accurately models complex binary and multiple systems, including relativistic effects, with efficient scaling and minimal performance loss at high binary fractions.

Contribution

The paper introduces BIFROST, an advanced simulation code that extends FROST to include hierarchical systems, relativistic effects, and efficient GPU scaling for realistic star cluster modeling.

Findings

Efficiently simulates star clusters with up to 100% binary fraction.

Maintains good performance scaling up to 40 million particles across GPUs.

Accurately models extreme cluster events like core collapse and black hole mergers.

Abstract

We present BIFROST, an extended version of the GPU-accelerated hierarchical fourth-order forward symplectic integrator code FROST. BIFROST (BInaries in FROST) can efficiently evolve collisional stellar systems with arbitrary binary fractions up to $f_\mathrm{bin}=100\%$ by using secular and regularised integration for binaries, triples, multiple systems or small clusters around black holes within the fourth-order forward integrator framework. Post-Newtonian (PN) terms up to order PN3.5 are included in the equations of motion of compact subsystems with optional three-body and spin-dependent terms. PN1.0 terms for interactions with black holes are computed everywhere in the simulation domain. The code has several merger criteria (gravitational-wave inspirals, tidal disruption events and stellar and compact object collisions) with the addition of relativistic recoil kicks for compact object mergers. We show that for systems with $N$ particles the scaling of the code remains good up to $N_\mathrm{GPU} \sim 40\times N / 10^6$ GPUs and that the increasing binary fractions up to 100 per cent hardly increase the code running time (less than a factor $\sim 1.5$). We also validate the numerical accuracy of BIFROST by presenting a number of star clusters simulations the most extreme ones including a core collapse and a merger of two intermediate mass black holes with a relativistic recoil kick.