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

I. Free-floating MLPs

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie, University of Heidelberg, Mönchhofstrasse 12–14, 69120 Heidelberg, Germany
² 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, P.R. China
³ Nicolaus Copernicus Astronomical Centre Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
⁴ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, HUN-REN CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
⁵ Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03143 Kyiv, Ukraine
⁶ Kavli Institute for Astronomy and Astrophysics, Peking University, Yiheyuan Lu 5, Haidian Qu, 100871 Beijing, China
⁷ Department of Astronomy, School of Physics, Peking University, Yiheyuan Lu 5, Haidian Qu, 100871 Beijing, China
⁸ National Astronomical Observatories and Key Laboratory of Computational Astrophysics, Chinese Academy of Sciences, 20A Datun Rd., Chaoyang District, 100101 Beijing, China

^★ Corresponding author; fmfd@uni-heidelberg.de

Received: 1 October 2024
Accepted: 9 December 2024

Abstract

Context. Low-mass bodies, such as comets, asteroids, planetesimals, and free-floating planets, are continuously injected into the intra-cluster environment after expulsion from their host planetary systems. These objects can be modelled as massless particles (MLPs). Notably, the dynamics of large populations of MLPs have received little attention in the literature.

Aims. We investigate the dynamical evolution of MLP populations in star clusters and characterise their kinematics and ejection rates.

Methods. We present NBODY6++GPU-MASSLESS, a modified version of the N-body simulation code NBODY6++GPU that allows for fast integration of star clusters that contain large numbers of MLPs. NBODY6++GPU-MASSLESS contains routines specifically directed at the dynamical evolution of low-mass bodies, such as planets.

Results. Unlike stars, MLPs do not participate in the mass segregation process. Instead, MLPs mostly follow the gravitational potential of the star cluster, which gradually decreases over time due to stellar ejections and stellar evolution. The dynamical evolution of MLPs is primarily affected by the evolution of the core of the star cluster. This is most apparent in the outer regions for clusters with higher initial densities. High escape rates of MLPs are observed before the core collapse, after which escape rates remain stable. Denser star clusters undergo a more intense core collapse, but this does not impact the dynamical evolution of MLPs. We find the speeds of escaping stars are similar to those of escaping MLPs when disregarding the high-velocity ejections of neutron stars during the first 50 Myr.