Low-velocity streams in the solar neighbourhood caused by the Galactic bar

Abstract

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<phi_0<45 [deg] 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 ~2 Gyr ago. This model argues against a common dynamical origin for the Hyades and Pleiades moving groups.