Non-resonant relaxation of anisotropic globular clusters
  Globular clusters are dense stellar systems whose core slowly contracts under
the effect of self-gravity. The rate of this process was recently found to be
directly linked to the initial amount of velocity anisotropy: tangentially
anisotropic clusters contract faster than radially anisotropic ones.
Furthermore, initially anisotropic clusters are found to generically tend
towards more isotropic distributions during the onset of contraction.
Chandrasekhar's "non-resonant" (NR) theory of diffusion describes this
relaxation as being driven by a sequence of local two-body deflections along
each star's orbit. We explicitly tailor this NR prediction to anisotropic
clusters, and compare it with $N$-body realisations of Plummer spheres with
varying degrees of anisotropy. The NR theory is shown to recover remarkably
well the detailed shape of the orbital diffusion and the associated initial
isotropisation, up to a global multiplicative prefactor which increases with
anisotropy. Strikingly, a simple effective isotropic prescription provides
almost as good a fit, as long as the cluster's anisotropy is not too strong.
For these more extreme clusters, accounting for long-range resonant relaxation
may be necessary to capture these clusters' long-term evolution.
