Principal Investigator Zdzislaw Meglicki Project n08

Centre for Information Science Research Machine VP

Co-Investigators Dayal T Wickramasinghe and Robert L Dewar

Department of Mathematics and

Research School of Physical Sciences and Engineering

MHD Phenomena in Accretion Systems

Our current investigation resulted in a construction of a dynamical model for the circumnuclear ring, which is quite close to the gc2 model of Wardle and Koenigl. At the same time we have obtained not only qualitative but also quantitative agreement with observations made by Serabyn et al. Furthermore, our model incorporates other interesting features observed in the Galactic centre, such as spiraling streamers, expansion of the ring, jets, and warping of the ring, none of which are covered by kinematic models of Wardle and Koenigl. The spiraling streamers detaching from the inner edge of the ring seem to be the manifestation of Hawley-Balbus instability occurring on a large scale. Our observations confirm the view that jets, and other signs of activity in accretion systems, especially in active galactic nuclei, are to a large extent driven by magnetohydrodynamics.

What are the basic questions addressed?

What is the basic topology of flows, currents, and magnetic fields that develops during the accretion event for various initial and boundary conditions?

What are the results to date and the future of the work?

We have carried out three large supercomputer simulations. The first one was concerned with the evolution of a young accretion disk in a compact binary system. The second simulation was concerned with the gravitational collapse of a magnetised vortex. The third simulation investigated the collapse of a vortex in a skewed ambient magnetic field. We plan to investigate interaction of accreting flows with magnetospheres of planets, white dwarfs and neutron stars. The results of all our investigations were either already published or submitted for publications in international refereed journals.

What computational techniques are used and why is a supercomputer required?

Smoothed Particle Magnetohydrodynamics is used to carry on the simulations. All simulations are 3 dimensional. This implies that very large data sets corresponding to at least half a million of Lagrangian nodes must be handled. At preset that type of work can be carried out on a supercomputer only. Depending on a computational system employed our SPMHD simulations on the VP are up to 11 times faster than comparable simulations for a comparable resolution run on a SPARCcentre 2000 using the NCSA Zeus3d finite differences code. We use the latter frequently in order to gauge the accuracy and stability properties of our numerical method.

Publications

Verification and Accuracy of Smoothed Particle Magnetohydrodynamics Z Meglicki, D Sitsky, Computer Physics Communications, 81, 91--104, 1994

Gravitational collapse of a magnetised vortex -- application to the Galactic centre Z Meglicki,

MNRAS, 272, 717--729, in press.

Computer Science Meets General Relativity, Z Meglicki, D Wickramasinghe, R L Dewar, IEEE Computational Science and Engineering, submitted

Application of weighted differences method to MHD, an alternative to SPH, Z Meglicki, Computer Physics Communications, submitted.