Journal Article

An analytical description of the evolution of binary orbital-parameter distributions in <i>N</i>-body computations of star clusters

Michael Marks, Pavel Kroupa and Seungkyung Oh

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 417, issue 3, pages 1684-1701
Published in print November 2011 | ISSN: 0035-8711
Published online October 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19257.x
An analytical description of the evolution of binary orbital-parameter distributions in N-body computations of star clusters

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A new method is presented to describe the evolution of the orbital-parameter distributions for an initially universal binary population in star clusters by means of the currently largest existing library of N-body models. It is demonstrated that a stellar-dynamical operator, ΩMecl, rhdyn(t), exists, which uniquely transforms an initial (t= 0) orbital-parameter distribution function for binaries, , into a new distribution, , depending on the initial cluster mass, Mecl, and half-mass radius, rh, after some time t of dynamical evolution. For distribution functions derived are used, which are consistent with constraints for pre-main-sequence and Class I binary populations. Binaries with a lower energy and a higher reduced mass are dissolved preferentially. The Ω operator can be used to efficiently calculate and predict binary properties in clusters and whole galaxies without the need for further N-body computations. For the present set of N-body models, it is found that the binary populations change their properties on a crossing time-scale such that ΩMecl, rhdyn(t) can be well parametrized as a function of the cluster density, ρecl. Furthermore, it is shown that the binary fraction in clusters with similar initial velocity dispersions follows the same evolutionary tracks as a function of the passed number of relaxation times. Present-day observed binary populations in star clusters put constraints on their initial stellar densities, ρecl, which are found to be in the range of 102≲ρecl(≤rh)/M pc−3≲ 2 × 105 for open clusters and a few ×103≲ρecl(≤rh)/M pc−3≲ 108 for globular clusters.

Keywords: methods: analytical; binaries: general; globular clusters: general; open clusters and associations: general; galaxies: star clusters: general

Journal Article.  13441 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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