Gas flow in barred potentials II. Bar Driven Spiral Arms
  Spiral arms that emerge from the ends of a galactic bar are important in
interpreting observations of our and external galaxies. It is therefore
important to understand the physical mechanism that causes them. We find that
these spiral arms can be understood as kinematic density waves generated by
librations around underlying ballistic closed orbits. This is even true in the
case of a strong bar, provided the librations are around the appropriate closed
orbits and not around the circular orbits that form the basis of the epicycle
approximation. An important consequence is that it is a potential's orbital
structure that determines whether a bar should be classified as weak or strong,
and not crude estimates of the potential's deviation from axisymmetry.
