Dynamical Evolution of Globular Clusters

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It is known that the cores of globular clusters evolve toward higher density states due to two-body gravitational interactions. When the densities of the cores become higher than a certain value, we have to take into account other interactions. We call the evolution of this stage post-collapse evolution. Investigations of post-collapse evolution of globular clusters become one of the most important problems in theoretical astrophysics.In this project we treated stars as point masses and neglected the effects of finite size of stars.In 1985 fiscal year we studied the evolution of cores of globular clusters with N-body numerical experiments. When the core densities of globular clusters become sufficiently high, hard binaries are formed inside the cores. The binaries interact with field stars and reversal of core contraction occurs. However, when the binding energies of the binaries become large enough, the binaries are ejected from the cores and the cores contract again. The densities of … More the cores become high again and hard binaries are formed. This leads to the reexpansion of the cores. Thus the oscillations of the core density were found to be due to the activity of hard binaries.In 1986 fiscal year N-body codes were vectorized with the collaboration of Dr. S.J. Aarseth, Institute of Astronomy, University of Cambridge. As the result, the computations with N-body code were speeded up more than ten times.Though N-body experiments are speeded up, a large amount of computational time is still necessary. Therefore it is impossible to follow the long-term evolution of globular clusters. We are attempting to follow the long-term evolution of cores of globular cluster by making simple models. We have known the probability of formation of binaries, the rate of interaction between binaries and field stars, and the rate of energy production. We are making models of cores of globular clusters with the use of these data. This investigation is under progress now and we have not got definite conclusions yet. Less