  The equilibration of hot and dense nuclear matter produced in the central
region in central Au+Au collisions at $\sqrt{s}=200$ AGeV is studied within the
microscopic transport model UrQMD. The pressure here becomes isotropic at $t
\approx 5$ fm/c. Within the next 15 fm/c the expansion of the matter proceeds
almost isentropically with the entropy per baryon ratio $S/A \approx 150$.
During this period the equation of state in the $(P,\epsilon)$-plane has a very
simple form, $P=0.15 \epsilon$. Comparison with the statistical model (SM) of
an ideal hadron gas reveals that the time of $\approx 20$ fm/$c$ may be too
short to attain the fully equilibrated state. Particularly, the fractions of
resonances are overpopulated in contrast to the SM values. The creation of such
a long-lived resonance-rich state slows down the relaxation to chemical
equilibrium and can be detected experimentally.
