Angular-momentum-induced phase transition in spherical gravitational systems: N-body simulations.

The role of thermodynamics in the evolution of systems evolving under purely gravitational forces is not completely established. Both the infinite range and singularity in the Newtonian force law preclude the use of standard techniques. However, astronomical observations of globular clusters suggest that they may exist in distinct thermodynamic phases. Here, using dynamical simulation, we investigate a model gravitational system that exhibits a phase transition in the mean-field limit. The system consists of rotating, concentric, mass shells of fixed angular-momentum magnitude and shares identical equilibrium properties with a three-dimensional point mass system satisfying the same condition. The mean-field results show that a global entropy maximum exists for the model, and a first order phase transition takes place between "quasi-uniform" and "core-halo" states, in both the microcanonical and canonical ensembles. Here we investigate the evolution and, with time averaging, the equilibrium properties of the isolated system. Simulations were carried out in the transition region, at the critical point, and in each clearly defined thermodynamic phase, and striking differences were found in each case. We find full agreement with mean-field theory when finite-size scaling is accounted for. In addition, we find that (1) equilibration obeys power-law behavior, (2) virialization, equilibration, and the decay of correlations in both position and time, are very slow in the transition region, suggesting that the system is also spending time in the metastable phase, and (3) there is a strong evidence of long-lived, collective oscillations in the supercritical region.