A COMPARISON BETWEEN SPH AND PPM - SIMULATIONS OF STELLAR COLLISIONS

A full 3D implementation of the smoothed particle hydrodynamics method (SPH) is compared to the 3D Eulerian piecewise parabolic method (PPM) . Both codes were used to simulate a collision between a 0.5M. main-se quence star (modelled as a n = 3/2 polytrope) and a white dwarf star o f equal mass (treated as a point mass). The encounter was simulated 6 times with SPH (the number of particles ranging between 2900 and 27 00 0) and twice with PPM (using 64(3) and 128(3) cartesian zones). A mult iple grid implementation of the PPM code was also employed (5 nested g rids of 32(3) zones each). Our results show that the key factor determ ining the overall result of the collision is the form of the potential well of the point mass. Depending on the exact numerical smoothing of the well, different trajectories, amounts of mass accretion and maxim um temperatures are obtained. Once both codes are run with the same po int mass potential, the global values (e.g. total energy, angular mome ntum, center of mass trajectory, etc.) are in good agreement. However the results differ in the values of local quantities like density dist ribution in the immediate vicinity of the point mass, and in the envel ope structure. The choice of an impartial test problem is difficult, s ince advantages and disadvantages of both methods are largely mutually exclusive. For the modeling of stellar collisions, SPH and PPM produc e results that compare quite favorably. Hence, given that both schemes are fundamentally different, we assume that common features are physi cal in nature and not numerical.