Single Stars

In many cases, the first thing to do with the SPH code is to calculate a single-star model, which can be used later either by itself or in a multi-body system. The SPH code contains two subroutines which create single stars. One uses equal-mass SPH particles, which are laid down Monte-Carlo style, based on a weighting function calculated for the density profile you choose. The other sets up a hexagonal close-packed array of particles, with constant grid spacing, but with particles of varying mass. The latter approach gives better resolution for the outer edge of each star, but can be vulnerable to spurious mass segregation within collision remnants, as heavier particles fall into the regions with the largest gravitational potential. For either case, we recommend you relax the model by setting the parameter NRELAX=1, yielding a damping equation exactly like the one above. Typically, you should calculate for several relaxation times before treating a configuration as relaxed.

To ensure that the configuration computed during relaxation will work properly during later calculations, we make sure to adjust the center of mass of the matter during each timestep so that it is located at the origin.