The 4:1 Outer Lindblad Resonance of a long slow bar as a potential
  explanation for the Hercules stream
  There are multiple groups of comoving stars in the Solar neighbourhood, which
can potentially be explained as the signatures of one of the fundamental
resonances of non-axisymmetric structure such as the Galactic bar or spiral
arms. One such stream, Hercules, has been proposed to result from the outer
Lindblad resonance (OLR) of a short fast rotating bar as shown analytically, or
the corotation resonance (CR) of a longer slower rotating bar as observed in an
N-body model. We show that by including an m = 4 Fourier component in an
analytical long bar model, with an amplitude that is typical for bars in N-body
simulations, we can reproduce a Hercules like feature in the kinematics of the
Solar neighbourhood. We then describe the expected symmetry in the velocity
distribution arising from such a model, which we will soon be able to test with
Gaia.
