Activities in 1993-1994

From the Report for Academic Year 1993-1994
of the Institute for Advanced Study

PIET HUT has studied several topics in the general area of dense stellar systems, using a variety of analytical and numerical techniques. During the Summer of 1993, he visited the Yukawa Institute for Fundamental Physics at the University in Kyoto. There he collaborated with Masataka Fukugita on a project to determine cross sections for merging encounters between galaxies, in order to determine the rate of galaxy merging in rich groups of galaxies.

During the Fall of 1993, Prof. Hut took part in a program at the Institute for Theoretical Physics in Santa Barbara, entitled `dynamics of dense stellar systems'. During this workshop, he developed a novel algorithm for accurate orbit integrations in the gravitational many-body problem, in collaboration with Jun Makino from Tokyo University, and Steve McMillan from Drexel University. Given any existing integration scheme, this new algorithm provides a prescription for iterative time symmetrization of the time step length. Even with only one iteration, this meta-algorithm typically gives an accuracy significantly higher than the original algorithm could provide for the same amount of computer time.

Another project, started at the workshop in Santa Barbara, involves a detailed study of three-body scattering processes for strongly unequal masses. The main applications are in star clusters, where double stars composed of relatively light main sequence stars may encounter neutron stars and black holes. Together with Steve McMillan and Douglas Heggie, from the University of Edinburgh, Prof. Hut completed the first general survey of unequal mass three-body scattering, presenting analytical expressions based on phase-space arguments together with numerical results to determine the overall scaling factors. Other applications of these scattering experiments were made in collaboration with Fred Rasio, currently a member in the School of Natural Sciences, by providing formation scenarios of triple star systems in globular clusters, following recent observational evidence in that direction.

In collaboration with a group at the astronomy department of Tokyo, led by Prof. Sugimoto, Prof. Hut continued the research projects using the special-purpose computers constructed in Tokyo. One of the first applications, in collaboration with Jun Makino, was a detailed survey of encounters between galaxies, to determine the merging criteria for a variety of models for galactic structure. Another ongoing project, which will use the higher-precision special-purpose hardware which is expected to become available the next year, involves the development of orbit integration software using the new algorithm mentioned above.