  We study the flow of a spinor (F=1) Bose-Einstein condensate in the presence
of an obstacle. We consider the cases of ferromagnetic and polar spin-dependent
interactions and find that the system demonstrates two speeds of sound that are
identified analytically. Numerical simulations reveal the nucleation of
macroscopic nonlinear structures, such as dark solitons and vortex-antivortex
pairs, as well as vortex rings in one- and higher-dimensional settings
respectively, when a localized defect (e.g., a blue-detuned laser beam) is
dragged through the spinor condensate at a speed larger than the second
critical speed.
