DYNAMICAL TESTS OF A PARTICLE-MESH COMPUTER SIMULATION MODEL WITH APPLICATIONS IN ASTROPHYSICS

The theory of Particle-Mesh simulation of astrophysical systems is presented in the first three chapters. Chapter 4 contains the results of extensive tests of our two-dimensional computer model on a circular binary system. These tests assisted us in obtaining the best mix of parameters, computational schemes and methods such as the potential computation method, mesh size, mass assignment and force interpolation scheme and timestep size. We found that our model could simulate small galactic systems quite successfully without much loss of accuracy (approximately 6% loss of total angular momentum per revolution). The Direct Summation and Fourier methods for potential computation gave more accurate simulation than the approximate method. However, as the Direct Summation method is computationally slower, it is impractical for simulation with large meshes. The most accurate mass assignment and force interpolation scheme found was the Cloud-In-Cell. The condition for stability (wDT ? 2) was also verified. Chapter S contains the results of similar tests of our computer model on a many-particle disk containing 2000 - 10000 particles. The results are consistent with those obtained in the two-particle experiments thus reconfirming the conclusions made previously. We found that the disk model is unstable and breaks up rapidly. We have also checked on the effects of using different pseudo-random number generators for obtaining the initial particle distribution in the disk. These tests revealed that the final simulation result is dependent on the quality of the pseudo-random generator used in the simulation. The experiments using built-in' symmetry of the initial particle distribution proved useful in testing the accuracy and reliability of our computer model. In chapter 6 we used our computer model to simulate the evolution of a galactic system containing a point massive core. The core stabilizes the disk and leads to the formation of spiral arms. These spirals are tighter when the mass of the core is larger. Further, the region surrounding the massive core is depleted of stars as these stars have higher velocities than those in the outer region. When the stable spiral system was perturbed by an approaching massive projectile such as a galactic-sized massive black hole, the disk became violently unstable and was totally dispersed. Similar behaviour was observed irregardless of the initial velocity of the projectile and the instance when it was introduced in the vicinity of the disk. The use of different core and projectile mass also did not inhibit this dispersive action.

Author dc:creator

• BENJAMIN LEE