A comparison between SPH and PPM: simulations of stellar collisions

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Hydrodynamics, Methods: Numerical, Stars: Evolution Stars: Kinematics

Scientific paper

A full 3D implementation of the smoothed particle hydrodynamics method (SPH) is compared to the 3D Eulerian piecewise parabolic method (P PM). Both codes were used to simulate a collision between a 0.5Msun main- sequence star (modelled as a n=3/2 polytrope) and a white dwarf star of equal mass (treated as a point mass). The encounter was simulated 6 times with SPH (the number of particles ranging between 2900 and 27000) and twice with PPM (using 643 and l283 cartesian zones). A multiple grid implementation of the PPM code was also employed (5 nested grids of 323 zones each).
Our results show that the key factor determining 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 maximum temperatures are obtained. Once both codes are run with the same point mass potential, the global values (e.g. total energy, angular momentum, center of mass trajectory, etc.) arc in good agreement. However the results differ in the values of local quantities like density distribution in the immediate vicinity of the point mass, and in the envelope structure.
The choice of an impartial test problem is difficult, since advantages and disadvantages of both methods are largely mutually exclusive. For the modeling of stellar collisions, SPH and PPM produce results that compare quite favorably. Hence, given that both schemes are fundamentally different, we assume that common features are physical in nature and not numerical.

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