Probing the Galaxy's bars via the Hercules stream

It has been suggested that a resonance between a rotating bar and stars in the solar neighbourhood can produce the so called ‘Hercules stream’. Recently, a second bar may have been identified in the Galactic centre, the so called ‘long bar’, which is longer and much flatter than the traditional Galactic bar and has a similar mass. We looked at the dynamical effects of both bars, separately and together, on orbits of stars integrated backwards from local position and velocities and a model of the Galactic potential which includes the bars directly. Both bars can produce Hercules like features and allow us to measure the rotation rate of the bar(s). We measure a pattern speed, for both bars, of 1.93 ± 0.03 times the local circular frequency. This is slightly higher than previous measurements for the Galactic bar, but is mainly because we adopt a slightly different solar motion. Finally, we identify a new kinematic feature in local velocity space, caused by the long bar, which is tempting to identify with the high-velocity ‘Arcturus’ stream.