SCYON Abstract

Received on September 5 2011

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

AuthorsMarks Michael (1,2,3), Kroupa Pavel (1), Oh Seungkyung (1,2,3)
Affiliation(1) Argelander Institut für Astronomie, Bonn, Germany
(2) Max-Planck-Institut für Radioastronomie, Bonn, Germany
(3) Member of the International Max-Planck Research School for Astronomy and Astrophysics at the Universities of Bonn and Cologne
Accepted byMonthly Notices of the Royal Astronomical Society
Contactmmarks@astro.uni-bonn.de
URLhttp://esoads.eso.org/abs/2011arXiv1106.5050M
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Abstract

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, Ωdyn, exists, which uniquely transforms an initial (t=0) orbital parameter distribution function for binaries, Din, into a new distribution, Dfin, depending on the initial cluster mass, Mecl, and half-mass radius, rh, after some time t of dynamical evolution. For Din the distribution functions derived by Kroupa (1995a,b) 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 Ωdyn can be well parametrized as a function of the cluster density, ρin. 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, ρin, which are found to be in the range 102~<ρin(≤rh)Mpc~<2x105 for open clusters and a few 103~<ρin(≤rh)Mpc~<108 for globular clusters, respectively.