On the Self-Consistent Response of Stellar Systems to Gravitational Shocks

We study the reaction of a globular star cluster to a time-varying tidal perturbation (gravitational shock) using self-consistent N-body simulations and address two questions. First, to what extent is the cluster interior protected by adiabatic invariants. Second, how much further energy change does the postshock evolution of the cluster potential produce and how much does it affect the dispersion of stellar energies. We introduce the adiabatic correction as ratio of the energy change, 〈∆E〉, to its value in the impulse approximation. When the potential is kept fixed, the numerical results for the adiabatic correction for stars with orbital frequency ω can be approximated as (1 + ωτ ). For shocks with the characteristic duration of the order the half-mass dynamical time of the cluster, τ ∼< tdyn,h, the exponent γ = 5/2. For more prolonged shocks, τ ∼> 4 tdyn,h, the adiabatic correction is shallower, γ = 3/2. When we allow for self-gravity and potential oscillations which follow the shock, the energy of stars in the core changes significantly, while the total energy of the system is conserved. Paradoxically, the postshock potential fluctuations reduce the total amount of energy dispersion, 〈∆E2〉. The effect is small but real and is due to the postshock energy change being statistically anti-correlated with the shock induced heating. These results are to be applied to Fokker-Planck models of the evolution of globular clusters. Subject headings: stellar dynamics — globular clusters: general — methods: numerical