Philip A. Hughes
Dept of Astronomy, University of Michigan
hughes@astro.lsa.umich.edu
Mark A. Miller
Dept.
of Physics, Washington University, St Louis
mamiller@void.wustl.edu
This raises both questions and possibilities: how can highly relativistic flows suffer significant curvature, and yet retain their integrity? It has been argued (Begelman, Blandford & Rees 1984) that sublight-year-scale flows are highly dissipative, and should radiate a significant fraction of their flow energy if subjected to a perturbation such as bending. How do transverse shocks propagate along a curved flow? Will the shock plane rotate with respect to the flow axis; will the shock strengthen or weaken? What role does flow curvature have to play in explaining phenomena such as stationary knots between which superluminal components propagate (Shaffer et al. 1987), knots which brighten after an initial fading (Mutel et al. 1990), and changing component speed (Lobanov & Zensus 1996). Numerous lines of evidence point convincingly to the occurrence of oblique shocks (e.g., Heinz & Begelman 1997; Lister & Marscher 1999; Marscher et al. 1997; Polatidis & Wilkinson 1998); how do they form and evolve? All these issues are amenable to study through hydrodynamic simulation - but all demand that such simulations be 3-D.
Furthermore, it has become evident over the last five years that such highly energetic flows are also found in Galactic objects with stellar mass `engines'. In the galactic superluminals GRS 1915+105 (Mirabel & Rodríguez 1994, 1995) and GRO J1655-40 (Hjellming & Rupen 1995; Tingay et al. 1995) the observed motions indicate jet flow speeds up to 92%c. There is compelling evidence from the observation of optical afterglow that gamma-ray bursts are of cosmological origin, and whether produced by the mergers of compact objects, or accretion induced collapse (AIC), simple relativistic fireball models seem ruled out, the data strongly favoring highly relativistic jets (Dar 1998). Thus a detailed understanding of the dynamics of collimated relativistic flows has wide application in astrophysics.
This page illustrates some of our initial explorations
of the key issues pertaining to the uniquely 3-D dynamics of relativistic
astrophysical flows.
Since 1993 we have used machines at OSC and elsewhere in a continuing project to study the dynamics and radiation properties of relativistic extragalactic jets. Our first results (Duncan & Hughes 1994) constituted the first-published high-resolution axisymmetric simulations of these objects. That work demonstrated the stability of highly relativistic flows, and suggested an interpretation of a long-known difference between two classes of astrophysical flows in terms of flow speed-dependent stability. That work was performed on a combination of computers, including workstations at BGSU, the University of Michigan, and the OSC Cray and OVL Onyx machines. In addition, OSC personnel helped us make movies of the 2-D simulations which proved very useful in both the assessment of the science and the communication of the work.
Some representative graphics illustrating the jet dynamics is shown in the following Fig.1.
The major focus of the present simulations currently using OSC resources are 3-D relativistic jet simulations using our Adaptive Mesh Refinement code. Over the last two years we have performed numerous tests on the code and have endeavored to make it as efficient as the underlying algorithms permit. Following this extensive development and testing, the code is ready to be used to perform research level simulations on the OSC Cray T94 and the Origin 2000. The computational requirements of such simulations are significant in both memory and wall clock hours, and the 3-D simulations shown here require facilities of the caliber of those at the OSC, principally due to the memory requirements. The fast, large memory machines such as the Cray T94 and the Origin 2000 are ideal for our purpose, because currently available workstations provide insufficient memory for us to achieve anything like adequate resolution while the use of massively parallel technology still presents serious challenges to AMR-type codes.
We have run a number of
3-D test runs and some jet simulations. These are illustrated
in Fig.
2.