An orbit-averaged generalized-Landau kinetic equation for the relaxation evolution of finite weakly-coupled star clusters: `Discreteness' stochastic acceleration and anti-normalization

In the relaxation evolution of finite weakly-coupled star clusters, stars undergo stochastic acceleration due to the discreteness' of the clusters (the finiteness of the total stellar number), in addition to fundamental two-body relaxation processes. The acceleration is the essential non-collective many-body relaxation process. However, existing works have never detailed the discreteness’ stochastic acceleration and the corresponding mathematical model, i.e., the generalized-Landau (g-Landau) kinetic equation for the stellar distribution function. The present paper shows the kinetic formulation of an orbit-averaged g-Landau equation in action-angle coordinates, beginning with Bogoliubox-Born-Green-Kirkwood-Yvon hierarchy. We show that only the g-Landau equation can satisfy the anti-normalization condition among existing approximated star-cluster kinetic equations if gravitational polarization is neglected. Accordingly, only the equation can correctly define the total energy and the total number of stars in phase space. It also holds the conservation laws and the H-theorem. We further show that stars undergoing the discreteness stochastic acceleration can hold these physical properties independently of stars experiencing the two-body relaxation. Lastly, we suggest that the stochastic acceleration tends to isotropize the stellar DF and to drive star clusters into a stationary state in the early relaxation-evolution stage before the two-body relaxation thermalizes stars.