We find that a steady state bar induces transient features at low velocities in the solar neighborhood velocity distribution due to the initial response of the disc , following the formation of the bar .
We associate these velocity streams with two quasi-periodic orbital families , librating around the stable x _ { 1 } ( 1 ) and x _ { 1 } ( 2 ) orbits near the bar ’ s outer Lindblad resonance ( OLR ) .
In a reference frame moving with the bar , these otherwise stationary orbits precess on a timescale dependent on the strength of the bar , consistent with predictions from a simple Hamiltonian model for the resonance .
This behavior allows the two orbital families to reach the solar neighborhood and manifest themselves as clumps in the u-v plane moving away from ( x _ { 1 } ( 2 ) ) , and toward ( x _ { 1 } ( 1 ) ) the Galactic center .
Depending on the bar parameters and time since its formation , this model is consistent with the Pleiades and Coma Berenices , or Pleiades and Sirius moving groups seen in the Hipparcos stellar velocity distribution , if the Milky Way bar angle is 30 ^ { \circ } \lesssim \phi _ { 0 } \lesssim 45 ^ { \circ } and its pattern speed is \Omega _ { b } / \Omega _ { 0 } = 1.82 \pm 0.07 , where \Omega _ { 0 } is the angular velocity of the local standard of rest ( LSR ) .
Since the process is recurrent , we can achieve a good match about every six LSR rotations .
However , to be consistent with the fraction of stars in the Pleiades , we estimate that the Milky Way bar formed \sim 2 Gyr ago .
This model argues against a common dynamical origin for the Hyades and Pleiades moving groups .