  The rotational motion of an interacting Bose-Einstein condensate confined by
a harmonic trap is investigated by solving the hydrodynamic equations of
superfluids, with the irrotationality constraint for the velocity field. We
point out the occurrence of an overcritical branch where the system can rotate
with angular velocity larger than the oscillator frequencies. We show that in
the case of isotropic trapping the system exhibits a bifurcation from an
axisymmetric to a triaxial configuration, as a consequence of the interatomic
forces. The dynamical stability of the rotational motion with respect to the
dipole and quadrupole oscillations is explicitly discussed.
