An analytical description of the evolution of binary orbitalparameter distributions in Nbody computations of star clusters
Abstract
A new method is presented to describe the evolution of the orbitalparameter distributions for an initially universal binary population in star clusters by means of the currently largest existing library of Nbody models. It is demonstrated that a stellardynamical operator, Ω^{Mecl, rh}_{dyn}(t), exists, which uniquely transforms an initial (t= 0) orbitalparameter distribution function for binaries, ?, into a new distribution, ?, depending on the initial cluster mass, M_{ecl}, and halfmass radius, r_{h}, after some time t of dynamical evolution. For ? distribution functions derived are used, which are consistent with constraints for premainsequence 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 Nbody computations. For the present set of Nbody models, it is found that the binary populations change their properties on a crossing timescale such that Ω^{Mecl, rh}_{dyn}(t) can be well parametrized as a function of the cluster density, ρ_{ecl}. 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. Presentday observed binary populations in star clusters put constraints on their initial stellar densities, ρ_{ecl}, which are found to be in the range of 10^{2}≲ρ_{ecl}(≤r_{h})/ M_{⊙} pc^{3}≲ 2 × 10^{5} for open clusters and a few ×10^{3}≲ρ_{ecl}(≤r_{h})/ M_{⊙} pc^{3}≲ 10^{8} for globular clusters.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 November 2011
 DOI:
 10.1111/j.13652966.2011.19257.x
 arXiv:
 arXiv:1106.5050
 Bibcode:
 2011MNRAS.417.1684M
 Keywords:

 methods: analytical;
 binaries: general;
 globular clusters: general;
 open clusters and associations: general;
 galaxies: star clusters: general;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 accepted for publication in MNRAS, 20 pages, 10 figures, 2 tables