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Theoretical Astrophysics
^ Numerical Simulations of Galaxy Collisions
S. A. Lamb*
Department of Energy, ASCI level 3 award

Computational exploration of galaxy collisions is conducted using massively parallel supercomputers and comparison of the simulations with observational data obtained at a variety of wavelengths to learn more of the physical conditions in the real systems. This is a collaborative effort with researchers at Lawrence Livermore National Laboratory, and it involves the construction of a new computational code which will also be useful for simulating a variety of astrophysically interesting situations, such as the formation of galaxies and the collisions of clouds in the ISM.

^ Active Galactic Nuclei, Dense Stellar Systems, and Galactic Environment
S. Lamb,* A. Baker (Cardiff, Wales), J. Perry (Cambridge, England)
University of Illinois

Researchers are investigating a self-consistent model on a large range of scales to understand the processes leading to nuclear activity in galaxies. Current observations support the view that interactions between galaxies may be crucial in triggering episodes of activity in some active galactic nuclei. Interactions also trigger some starbursts, and the team is investigating the relationship between these two phenomena. Researchers employ numerical simulations of colliding galaxies and analytical studies of the physics of the central regions of galaxies to obtain a detailed model that can be compared to observations of these systems.

^ An AXAF Investigation of the Archetypal ULIRG: Arp 220
S. A. Lamb;* D. Clements, A. Baker (Cardiff, Wales); K. Borne (NASA Goddard Space Flight Center); L. Colina (European Space Agency, Spain); J. McDowell (Smithsonian Center for Astrophysics, Harvard); C. Mundell (Manchester, England)
National Aeronautics and Space Administration

There are three primary scientific goals: to examine the role of the hot interstellar medium in Arp 220 and to see how it relates to the star formation regions and dust structures revealed by Hubble Space Telescope optical images; to search for any evidence of an active galactic nucleus (AGN) in the merging galaxy system Arp 220; and to examine the properties of the extended galactic superwind and look for any signs of interaction between it and the intergalactic medium.

^ Coalescence of Binary Black Holes and Neutron Stars: Computational Contributions to LIGO
S. L. Shapiro,* T. W. Baumgarte*
National Science Foundation PHY99-02833

Gravitational wave forms from the final plunge and coalescence of black hole and neutron star binaries are primary targets for LIGO and other gravitational wave interferometers. These waveforms can only be predicted by large-scale numerical computations. A stable and accurate code for evolving Einstein's equations in three spatial dimensions and with no special symmetries assumed is needed to produce binary coalescence simulations. Researchers will construct such a code to solve the problem of coalescing compact binaries, combining theory and computation in a symbiotic fashion to make the most rapid progress possible toward the goal of producing gravitational wave forms.

^ Rotating and Binary Stars in General Relativity
S. L. Shapiro
National Aeronautics and Space Administration, NAG5-10781

The existence of binary and rotating neutron stars is beyond dispute. However, the two-body problem remains the most important unsolved problem in classical general relativity. The solution has important consequences for the detection of gravitational waves by laser interferometers now under construction and for resolving other astrophysical puzzles, such as the origin of gamma-ray bursts and planets around pulsars. Likewise, the dynamical stability of rotating neutron stars is not fully understood, and the final fate of unstable stars is not known. Similarly, the collapse of an unstable, rotating supermassive star has not been studied in general relativity, yet it remains a prime candidate for the formation of a supermassive black hole. Recent advances in this project have made it possible for researchers to solve these fundamental, closely related computational problems, essentially for the first time.

^ Supernova Fallback and the Emergence of Black Holes and Neutron Stars
S. L. Shapiro*
National Aeronautics and Space Administration, NAG5-8418

Researchers are investigating the fallback of gaseous material in a supernova onto the central compact object formed in the aftermath of the explosion. A goal of the project is to construct a radiation-hydrodynamics code to calculate the light curve and the spectrum from the supernova remnant, taking into account the luminosity produced by accretion onto the compact object. Researchers will determine to what extent the late-time behavior of the computed light curve is capable of revealing the presence of the compact object and whether the light curve can distinguish an accreting black hole from a neutron star. Preliminary calculations for SN 1997D suggest that a black hole may emerge above the emission of the envelope in just a few years. Its detection by the Hubble Space Telescope would thus provide unmistakable evidence for the presence of a black hole.

^ Theoretical Studies in Gravitation and Astrophysics
S. L. Shapiro*
National Science Foundation, PHY 00-90310

This project will enable continued research in general relativity and theoretical astrophysics addressing several problems involving general relativity, the generation of gravitational radiation, and relativistic hydrodynamics, radiative transport, and stellar dynamics. A common thread uniting different theoretical topics is the crucial role of gravitation, especially relativistic gravitation as described by Einstein's field equations of general relativity. Some of the topics for investigation include the inspiral and coalescence of binary neutron stars and black holes; the generation of gravitational waves from binaries and other promising astrophysical sources of gravitational radiation; gravitational collapse; the stability of rotating neutron stars and supermassive stars and the final fate of unstable stars; and the formation of supermassive black holes in the cores of galaxies and quasars. The approach will involve large-scale computations on parallel machines, as well as analytical modeling.


Summary of Engineering Research