8 - Initial setup  pp. 120-140

Initial setup

By Sverre J. Aarseth

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Introduction

A variety of procedures need to be carried out before the calculation proper can begin. The prescriptions for input parameters and options are discussed in chapter 7. Here we concentrate on different types of initial conditions for star cluster simulations, whereas planetary systems are described elsewhere. The cluster models are first generated for single stars with a specified initial mass function (hereafter IMF) and scaled to internal units. Since a variety of distributions may be considered, we provide several detailed algorithms. Next we present some procedures for including a realistic distribution of primordial binaries. Modelling of star clusters also requires external effects to be added. We distinguish between the motion of open clusters in circular orbits and globular clusters in 3D, with the galactic tidal force truncating the outer parts. Interstellar clouds form another perturbing agent which may be taken into account. Finally, with these procedures completed, the force polynomials for direct solutions as well as for any dominant two-body motions can be initialized.

Initial conditions for clusters

Although the choice of starting configurations for star cluster simulations is extremely wide, we may be guided by certain principles and observational constraints. Such models are usually represented by a smooth IMF and centrally concentrated density distribution. Depending on the objectives, the velocities may represent approximate equilibrium or initial collapse, whereas cosmological models are characterized by expansion.