Reversible time-step adaptation for the integration of few-body systems

TL;DR

This paper introduces a reversible, smooth time-step adaptation method for N-body simulations, improving energy conservation and robustness in chaotic few-body systems by comparing multiple criteria and symmetrisation techniques.

Contribution

It presents a novel weighted averaging approach for pairwise time steps and evaluates 27 criteria, enhancing stability and accuracy in complex gravitational systems.

Findings

Harmonic symmetrisation methods are most robust.

Weighted averaging improves energy conservation.

New weighting method slightly outperforms others.

Abstract

The time step criterion plays a crucial role in direct N-body codes. If not chosen carefully, it will cause a secular drift in the energy error. Shared, adaptive time step criteria commonly adopt the minimum pairwise time step, which suffers from discontinuities in the time evolution of the time step. This has a large impact on the functioning of time step symmetrisation algorithms. We provide new demonstrations of previous findings that a smooth and weighted average over all pairwise time steps in the N-body system, improves the level of energy conservation. Furthermore, we compare the performance of 27 different time step criteria, by considering 3 methods for weighting time steps and 9 symmetrisation methods. We present performance tests for strongly chaotic few-body systems, including unstable triples, giant planets in a resonant chain, and the current Solar System. We find that the harmonic symmetrisation methods (methods A3 and B3 in our notation) are the most robust, in the sense that the symmetrised time step remains close to the time step function. Furthermore, based on our Solar System experiment, we find that our new weighting method based on direct pairwise averaging (method W2 in our notation), is slightly preferred over the other methods.