When does a star cluster become a multiple star system?-I. Lifetimes of equal-mass small-N systems

Nathan W.C. Leigh*, Michael M. Shara, Aaron M. Geller

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

8 Scopus citations


What is the difference between a long-lived unstable (or quasi-stable) multiple star system and a bona fide star cluster? In this paper, we present a possible framework to address this question, by studying the distributions of disruption times for chaotic gravitational encounters as a function of the number of interacting particles. To this end, we perform a series of numerical scattering experiments with the FEWBODY code, to calculate the distributions of disruption times as a function of both the particle number N and the virial coefficient k. The subsequent distributions are fit with a physically motivated function, consisting of an initial exponential decay followed by a very slowly decreasing tail at long encounter times due to long-lived quasi-stable encounters. We find three primary features characteristic of the calculated distributions of disruption times. These are as follows: (1) the system half-life increases with increasing particle number, (2) the fraction of long-lived quasi-stable encounters increases with increasing particle number and (3) both the system half-life and the fraction of quasi-stable encounters increase with decreasing virial coefficient. We discuss the significance of our results for collisional dynamics, and consider the extrapolation of our results to larger- N systems. We suggest that this could potentially offer a clear and unambiguous distinction between star clusters and (stable or quasi-stable) multiple star systems. Although we are limited by very-small-number statistics, our results tentatively suggest that (for our assumptions) this transition occurs at a critical particle number of order 100.

Original languageEnglish (US)
Pages (from-to)1242-1247
Number of pages6
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Jun 21 2016


  • Binaries: close
  • Globular clusters: general
  • Gravitation
  • Methods: analytical
  • Scattering
  • Stars: kinematics and dynamics

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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