arXiv subjects

Kerwann Tep

Publications and source records attributed to Kerwann Tep.

9 recordsLinked to original sources

arXiv2026

Modeling Globular Cluster Stellar Streams with a Basis-Expansion N-body Code

Globular cluster stellar streams probe galaxy-formation processes and can potentially reveal the distribution of dark matter in galaxies. In many theoretical studies, streams are modeled with particle-spray or direct N-body codes. But particle-spray methods abstract away the internal dynamics of the progenitor by making strong assumptions about the escape physics, while direct N-body is prohibitively expensive for realistic (N>10^5) systems. In this paper, we present the stream-modeling capabilities of KRIOS, a new basis-expansion N-body code for collisional stellar dynamics, that bridges this runtime vs. accuracy gap. We show that KRIOS reproduces NBODY6++GPU cluster models, and their associated streams, more accurately than particle spray in a fraction of the NBODY6++GPU wall-clock time. We then compare KRIOS to various particle-spray methods on 10 orbits similar to known Milky Way streams. The morphology and kinematics of these streams most disagree when the progenitor is tightly bound to the host, as these systems are often subject to stronger tidal forces. Finally, we discuss which elements of the progenitor physics are most important for modeling stellar streams and how these might be incorporated into particle-spray methods.

astro-ph.GA↗

arXiv2026

Very long-term relaxation of harmonic 1D self-gravitating systems

One-dimensional self-gravitating systems admit genuine thermodynamical equilibria. For systems with strictly monotonic orbital frequency profile, the Landau and Balescu-Lenard theories predict a relaxation time scaling linearly with the number of particles, $N$, in agreement with simulations. Yet, these theories become ill-posed for degenerate frequency profiles, as is the case in the harmonic potential, where all particles share the exact same mean orbital frequency. Using an exact collision-driven 1D integrator, we investigate numerically the self-consistent relaxation of 1D harmonic self-gravitating systems. We show that harmonic systems relax on a timescale that grows quadratically with $N$. We show that systems that are only partially degenerate display the same quadratic scaling for low $N$, but transition to the linear, non-degenerate behaviour for larger $N$. The larger the fraction of degenerate orbits, the larger the value of $N$ at which this transition of dynamical regime occurs. Finally, we explore the dynamics of fully non-degenerate systems, albeit with finite radial support: we confirm that their relaxation time scales linearly with $N$, though with a substantially larger prefactor than in non-compact systems. Astrophysically, this investigation should offer some new clues on the dynamics of density cores, as in the center of dwarf galaxies.

astro-ph.GA↗

arXiv2026

Discrepancies between Chandrasekhar's theory of relaxation and $N$-body simulations

Globular clusters are systems which are known to be particularly well described by two-body relaxation. In recent decades many studies have shown that Chandrasekhar's orbit-averaged theory is able to reproduce many features of numerical simulations. However, it has been claimed that differences between the theory and simulation remain, such as an amplitude mismatch of the rate of change of the distribution function. In this paper we compare the theoretical predictions of Chandrasekhar's theory for anisotropic clusters to precise $N$-body data. We show that more careful $N$-body measurements are able to reduce the claimed mismatch. Nevertheless, we observe a dependency of the remaining mismatch on both position and anisotropy. While the dependence on anisotropy may be understood qualitatively on theoretical grounds, the radial dependence implies that spatial inhomogeneities, and therefore collective effects, may become unavoidable to resolve the residual mismatch between theory and simulations.

astro-ph.GA↗

arXiv2025

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

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 Hénon - 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 Hénon'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.

astro-ph.GA↗

arXiv2025

Linear response of rotating and flattened stellar clusters: the oblate Kuzmin-Kutuzov Stäckel family

This paper investigates the linear response of a series of spheroidal stellar clusters, the Kuzmin-Kutuzov Stäckel family, which exhibit a continuous range of flattening and rotation, extending from an isochrone sphere to a Toomre disk. The method successfully replicates the growing modes previously identified in published $N$-body simulations. It relies on the efficiency of the matrix method to quantify systematically the effects of rotation and flattening on the eigenmodes of the galaxy. We identify two types of bi-symmetric instabilities for the flatter models - the so-called bending and bar-growing modes - the latter of which persists even for very round models. As anticipated, in its least unstable configurations, the system becomes flatter as its rotational speed increases. More realistic equilibria will be required to achieve a better match to the main sequence of fast-slow rotators. The corresponding code is made public.

astro-ph.GA↗

arXiv2024

Non-resonant relaxation of rotating globular clusters

The long-term relaxation of rotating, spherically symmetric globular clusters is investigated through an extension of the orbit averaged Chandrasekhar non-resonant formalism. A comparison is made with the long-term evolution of the distribution function in action space, measured from averages of sets of $N$-body simulations up to core collapse. The impact of rotation on in-plane relaxation is found to be weak. In addition, we observe a clear match between theoretical predictions and $N$-body measurements. For the class of rotating models considered, we find no strong gravo-gyro catastrophe accelerating core collapse. Both kinetic theory and simulations predict a reshuffling of orbital inclinations from overpopulated regions to underpopulated ones. This trend accelerates as the amount of rotation is increased. Yet, for orbits closer to the rotational plane, the non-resonant prediction does not reproduce numerical measurements. We argue that this mismatch stems from these orbits' coherent interactions, which are not captured by the non-resonant formalism that only addresses local deflections.

astro-ph.GA↗

arXiv2023

Predicting the linear response of self-gravitating stellar spheres and discs with LinearResponse.jl

We present LinearResponse.jl, an efficient, versatile public library written in julia to compute the linear response of self-gravitating (3D spherically symmetric) stellar spheres and (2D axisymmetric razor-thin) discs. LinearResponse.jl can scan the whole complex frequency plane, probing unstable, neutral and (weakly) damped modes. Given a potential model and a distribution function, this numerical toolbox estimates the modal frequencies as well as the shapes of individual modes. The libraries are validated against a combination of previous results for the spherical isochrone model and Mestel discs, and new simulations for the spherical Plummer model. Beyond linear response theory, the realm of applications of LinearResponse.jl also extends to the kinetic theory of self-gravitating systems through a modular interface.

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arXiv2022

Non-resonant relaxation of anisotropic globular clusters

Globular clusters are dense stellar systems whose core slowly contracts under the effect of self-gravity. The rate of this process was recently found to be directly linked to the initial amount of velocity anisotropy: tangentially anisotropic clusters contract faster than radially anisotropic ones. Furthermore, initially anisotropic clusters are found to generically tend towards more isotropic distributions during the onset of contraction. Chandrasekhar's "non-resonant" (NR) theory of diffusion describes this relaxation as being driven by a sequence of local two-body deflections along each star's orbit. We explicitly tailor this NR prediction to anisotropic clusters, and compare it with $N$-body realisations of Plummer spheres with varying degrees of anisotropy. The NR theory is shown to recover remarkably well the detailed shape of the orbital diffusion and the associated initial isotropisation, up to a global multiplicative prefactor which increases with anisotropy. Strikingly, a simple effective isotropic prescription provides almost as good a fit, as long as the cluster's anisotropy is not too strong. For these more extreme clusters, accounting for long-range resonant relaxation may be necessary to capture these clusters' long-term evolution.

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arXiv2021

Mapping the Galactic centre's dark cluster via Resonant Relaxation

Supermassive black holes in the centre of galaxies dominate the gravitational potential of their surrounding stellar clusters. In these dense environments, stars follow nearly Keplerian orbits, which get slowly distorted as a result of the potential fluctuations generated by the stellar cluster itself as a whole. In particular, stars undergo a rapid relaxation of their eccentricities through both resonant and non-resonant processes. An efficient implementation of the resonant diffusion coefficients allows for detailed and systematic explorations of the parameter space describing the properties of the stellar cluster. In conjunction with recent observations of the S-cluster orbiting SgrA*, this framework can be used to jointly constrain the distribution of the unresolved, old, background stellar cluster and the characteristics of a putative dark cluster. Specifically, we show how this can be used to estimate the typical mass and cuspide exponent of intermediate-mass black holes consistent with the relaxed state of the distribution of eccentricities in the observed S-cluster. This should prove useful in constraining super massive black hole formation scenarios.

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