Fast Multipole Methods for $N$-body Simulations of Collisional Star Systems

TL;DR

This paper introduces an optimized Fast Multipole Method (FMM) implementation, Taichi, enabling accurate, large-scale collisional star cluster simulations that are computationally more efficient than traditional direct $N$-body methods.

Contribution

The paper adapts a tree-based FMM with error control and acceleration techniques for collisional $N$-body simulations, achieving accuracy comparable to direct summation for large $N$ and improving computational efficiency.

Findings

Taichi achieves direct-summation accuracy for $N>10^4$.

The code accurately models collisional effects like dynamical friction.

Taichi outperforms other CPU-based codes in efficiency for large systems.

Abstract

Direct $N$-body simulations of star clusters are accurate but expensive, largely due to the numerous $\mathcal{O} (N^2)$ pairwise force calculations. To solve the post-million-body problem, it will be necessary to use approximate force solvers, such as tree codes. In this work, we adapt a tree-based, optimized Fast Multipole Method (FMM) to the collisional $N$-body problem. The use of a rotation-accelerated translation operator and an error-controlled cell opening criterion leads to a code that can be tuned to arbitrary accuracy. We demonstrate that our code, Taichi, can be as accurate as direct summation when $N> 10^4$. This opens up the possibility of performing large-$N$, star-by-star simulations of massive stellar clusters, and would permit large parameter space studies that would require years with the current generation of direct summation codes. Using a series of tests and idealized models, we show that Taichi can accurately model collisional effects, such as dynamical friction and the core-collapse time of idealized clusters, producing results in strong agreement with benchmarks from other collisional codes such as NBODY6++GPU or PeTar. Parallelized using OpenMP and AVX, Taichi is demonstrated to be more efficient than other CPU-based direct $N$-body codes for simulating large systems. With future improvements to the handling of close encounters and binary evolution, we clearly demonstrate the potential of an optimized FMM for the modeling of collisional stellar systems, opening the door to accurate simulations of massive globular clusters, super star clusters, and even galactic nuclei.