MYRIAD: a new N-body code for simulations of star clusters

Aims: We present a new C++ code for collisional N-body simulations of star clusters.
Methods: The code uses the Hermite fourth-order scheme with block time steps, to advance the particles in time, while the forces and neighboring particles are computed using the GRAPE-6 board. Special treatment is used for close encounters, and binary or multiple subsystems that form either dynamically or exist in the initial configuration. The structure of the code is modular and allows the appropriate treatment of more physical phenomena, such as stellar and binary evolution, stellar collisions, and evolution of close black-hole binaries. Moreover, it can be easily modified so that the part of the code that uses GRAPE-6, could be replaced by another module that uses other accelerating-hardware such as the graphics processing units (GPUs).
Results: Appropriate choice of the free parameters give a good accuracy and speed for simulations of star clusters up to and beyond core collapse. Simulations of Plummer models consisting of equal-mass stars reached core collapse at t ≃ 17 half-mass relaxation times, which compares very well with existing results, while the cumulative relative error in the energy remained below 10^-3. Comparisons with published results of other codes for the time of core collapse for different initial conditions, show excellent agreement. Simulations of King models with an initial mass-function, similar to those found in the literature, reached core collapse at t ≃ 0.17 ± 0.05 half-mass relaxation times, which is slightly earlier than expected from previous work. Finally, the code accuracy becomes comparable to and even better than the accuracy of existing codes, when a number of close binary systems is dynamically created in a simulation. This is because of the high accuracy of the method that is used for close binary and multiple subsystems.
Conclusions: The code can be used for evolving star clusters containing equal-mass stars or star clusters with an initial mass function (IMF) containing an intermediate mass black hole (IMBH) at the center and/or a fraction of primordial binaries, which are systems of particular astrophysical interest.