Dynamical evolution of massless particles in star clusters with NBODY6++GPU-MASSLESS

II. The long-term evolution of free-floating comets

¹ Department of Physics, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
² Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
³ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstrasse 12-14, 69120, Heidelberg, Germany
⁴ Dipartimento di Fisica, Sapienza, Universitá di Roma, P.le Aldo Moro, 5, 00185 Rome, Italy
⁵ Department of Physics, School of Mathematics and Physics, Xi’an Jiaotong-Liverpool University, 111 Ren’ai Road, Suzhou Dushu Lake Science and Education Innovation District, Suzhou Industrial Park, Suzhou 215123, PR China
⁶ Nicolaus Copernicus Astronomical Centre Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
⁷ Szechenyi Istvan University, Space Technology and Space Law Research Center, 9026 Gyor, Egyetem ter 1., Hungary
⁸ Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03143 Kyiv, Ukraine
⁹ National Astronomical Observatories and Key Laboratory of Computational Astrophysics, Chinese Academy of Sciences, 20A Datun Rd., Chaoyang District, 100101 Beijing, China
¹⁰ Kavli Institute for Astronomy and Astrophysics, Peking University, Yiheyuan Lu 5, Haidian Qu, 100871 Beijing, China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 20 September 2025
Accepted: 12 December 2025

Abstract

Context. Comets, asteroids, planetesimals, free-floating planets, and brown dwarfs are continuously injected into the intracluster environment after expulsion from their host-planetary systems or binary system. The dynamics of large populations of such free-floating comets (ffcs) in a star cluster environment is not yet fully understood.

Aims. We investigated the dynamical evolution of comet populations in star clusters and characterized the kinematics and ejection rates of ffc in a star cluster. Moreover, we determined whether a different initial energy distribution affects the mass segregation of the less massive population.

Methods. We carried out simulations using the N-body code NBODY6++GPU-MASSLESS, which allows the fast integration of star clusters that contain large numbers of massless particles, to characterize the dynamics of populations of low-mass particles with sub-virial and super-virial distributions.

Results. Comets do not participate in the mass-segregation process, similarly to planet-sized objects, regardless of their initial energy distribution. The latter slightly changed the whole dynamical evolution at the start of the simulation. We only observe an initial relaxation or collapse of the objects for super-virial and sub-virial ratios, respectively. The external regions of the ffcs population tend to be pulled back in the cluster core at the end of the simulation, suggesting the gravitational pull of the stars is pulling them back in the core. This phenomenon occurs at later times if the system is in virial equilibrium. Compared to less massive bodies, brown dwarfs experience more mass segregation. The inner regions tend to be more mixed with the stellar population.