  It is shown that data on strange particle production as a function of
centrality in Au-Au collisions at \sqrt(s)_{NN}= 200 GeV can be explained with
a superposition of emission from a hadron gas at full chemical equilibrium
(core) and from nucleon-nucleon collisions at the boundary (corona) of the
overlapping region of the two colliding nuclei. This model nicely accounts for
the enhancement of phi meson and strange particle production as a function of
centrality observed in relativistic heavy ion collisions at that energy. The
enhancement is mainly a geometrical effect, that is the increasing weight of
the core with respect to corona for higher centrality, while strangeness
canonical suppression in the core seems to play a role only in very peripheral
collisions. This model, if confirmed at lower energy, would settle the
long-standing problem of strangeness under-saturation in relativistic heavy ion
collisions, parametrized by $\gs$. Furthermore, it would give a unique tool to
locate the onset of deconfinement in nuclear collisions both as a function of
energy and centrality if this is to be associated to the onset of the formation
of a fully equilibrated core.
