KRIOS: A new basis-expansion $N$-body code for collisional stellar dynamics

TL;DR

KRIOS is a hybrid N-body simulation code that efficiently models collisional stellar systems, capturing complex features like triaxiality and rotation, and accurately reproduces key dynamical phenomena over long timescales.

Contribution

The paper introduces KRIOS, a novel hybrid N-body code combining self-consistent field methods with Hénon's collisional approach, enabling detailed 3D modeling of star clusters beyond previous limitations.

Findings

Accurately models long-term evolution of star clusters.

Reproduces core collapse and radial-orbit instability phenomena.

Captures non-spherical effects in stellar dynamics.

Abstract

The gravitational $N$-body problem is a nearly universal problem in astrophysics which, despite its deceptive simplicity, still presents a significant computational challenge. For collisional systems such as dense star clusters, the need to resolve individual encounters between $N$ stars makes the direct summation of forces - with quadratic complexity - almost infeasible for systems with $N\gtrsim 10^6$ particles over many relaxation times. At the same time, the most common Monte Carlo $N$-body algorithm - that of H\'enon - assumes the cluster to be spherically symmetric. This greatly limits the study of many important features of star clusters, including triaxiality, rotation, and the production of tidal debris. In this paper, we present a new hybrid code, KRIOS, that combines 3D collisionless relaxation using an adaptive self-consistent field method with collisional dynamics handled via H\'enon's method. We demonstrate that KRIOS can accurately model the long-term evolution of clusters and provide its complete phase-space information over many relaxation times. As a test of our new code, we present detailed comparisons to well-known results from stellar dynamics: (i) the collisional evolution of a family of Plummer spheres with varying anisotropy and rotation to core collapse, and (ii) the emergence of the radial-orbit instability in radially anisotropic star clusters, including its non-spherical effects.