Runaway collisions in young star clusters - I. Methods and tests

Marc Freitag*, Frederic A. Rasio, Holger Baumgardt

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

63 Scopus citations


We present the methods and preparatory work for our study of the collisional runaway scenario to form a very massive star (VMS, M* > 400 M ) at the centre of a young, compact stellar cluster. In the first phase of the process, a very dense central core of massive stars (M* ≃ 30-120 M⊙) forms through mass segregation and gravothermal collapse. This leads to a collisional stage, likely to result in the formation of a VMS (itself a possible progenitor for an intermediate-mass black hole) through a runaway sequence of mergers between the massive stars. In this paper, we present the runaway scenario in a general astrophysical context. We then explain the numerical method used to investigate it. Our approach is based on a Monte Carlo code to simulate the stellar dynamics of spherical star clusters, using a very large number of particles (a few 10 5 to several 10 6). Finally, we report on test computations carried out to ensure that our implementation of the important physics is sound. In a second paper, we present results from more than 100 cluster simulations realized to determine the conditions leading to the collisional formation of a VMS and the characteristics of the runaway sequences.

Original languageEnglish (US)
Pages (from-to)121-140
Number of pages20
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - May 2006


  • Galaxies: nuclei
  • Galaxies: star clusters
  • Galaxies: starburst
  • Methods: N-body simulations
  • Stars: formation
  • Stellar dynamics

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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