Rapid Mass segregation in small stellar clusters
  In this paper we focus our attention on small-to-intermediate N-body systems
that are, initially, distributed uniformly in space and dynamically cool
(virial ratios $Q=2T/|\Omega|$ below ~0.3). In this work, we study the mass
segregation that emerges after the initial violent dynamical evolution. At this
scope, we ran a set of high precision N-body simulations of isolated clusters
by means of HiGPUs, our direct summation N-body code. After the collapse, the
system shows a clear mass segregation. This (quick) mass segregation occurs in
two phases: the first shows up in clumps originated by sub-fragmentation before
the deep overall collapse; this segregation is partly erased during the deep
collapse to re-emerge, abruptly, during the second phase, that follows the
first bounce of the system. In this second stage, the proper clock to measure
the rate of segregation is the dynamical time after virialization, which (for
cold and cool systems) may be significantly different from the crossing time
evaluated from initial conditions. This result is obtained for isolated
clusters composed of stars of two different masses (in the ratio $m_h/m_l=2$),
at varying their number ratio, and is confirmed also in presence of a massive
central object (simulating a black hole of stellar size). Actually, in stellar
systems starting their dynamical evolution from cool conditions, the fast mass
segregation adds to the following, slow, secular segregation which is
collisionally induced. The violent mass segregation is an effect persistent
over the whole range of N ($128 \leq N \leq 1024$) investigated, and is an
interesting feature on the astronomical-observational side, too. The
semi-steady state reached after virialization corresponds to a mass segregated
distribution function rather than that of equipartition of kinetic energy per
unit mass as it should result from violent relaxation.
