  The Nambu-Goldstone mode, associated with vortex nucleation in a harmonically
confined, two-dimensional dilute Bose-Einstein condensate, is identified with
the lowest-lying envelope of octupole-mode branches, which are separated from
each other by the admixture of quadrupolar excitations. As the vortex
approaches the center of the condensate and the system's axisymmetry is
restored, the Nambu-Goldstone mode becomes massive due to its coupling to
higher rotational bands.
