How do binaries affect the derived dynamical mass of a star cluster?

M. B. N. Kouwenhoven, R. de Grijs

Research output: Contribution to journalArticle

5 Citations (Scopus)


The dynamical mass of a star cluster can be derived from the virial theorem, using the measured half-mass radius and line-of-sight velocity dispersion of the cluster. However, this dynamical mass may be a significant overestimation of the cluster mass if the contribution of the binary orbital motion is not taken into account. Here, we describe the mass overestimation as a function of cluster properties and binary population properties, and briefly touch on the issue of selection effects. We find that for clusters with a measured velocity dispersion of σlos≳ 10 km s-1 the presence of binaries does not affect the dynamical mass significantly. For clusters with σlos≲ 1 kms-1 (i.e., lowdensity clusters), the contribution of binaries to σlos is significant, and may result in a major dynamical mass overestimation. The presence of binaries may introduce a downward shift of Δlog(LV /Mdyn) = 0.05-0.4 (in solar units) in the log(LV /Mdyn) versus age diagram.

Original languageEnglish
Pages (from-to)171-176
Number of pages6
JournalAstrophysics and Space Science
Issue number2-4
Publication statusPublished - Nov 2009
Externally publishedYes


  • star clusters: general
  • methods: numerical
  • binaries: general

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