Softening in N-body simulations of collisionless systems

Abstract

The standard Plummer softening force law is physically inconsistent: theforce on a particle is calculated as if it were a point mass, but itscontribution to the gravitational field for other particles is that ofan extended mass. The use of uniform density spheres as models for theparticles leads to a consistent force and is physically more reasonable.For a King model with W(0) = 5, the optimal size of uniform densityspheres is approximately three times the optimal Plummer softeninglength. This allows a larger time step to be used and thus uniformdensity sphere softening is more efficient. The relaxation rate issmaller for the optimal uniform density sphere softening.