Probing the Galaxy's bars via the Hercules stream

ABSTRACT It has been suggested that a resonance between a rotating bar and stars in thesolar neighbourhood can produce the so called ‘Hercules stream’. Recently, a secondbar may have been identified in the Galactic centre, the so called ‘long bar’, whichis 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 orbitsof stars integrated backwards from local position and velocities, and a model of theGalactic potential which includes the bars directly. Both bars can produce Herculeslike features, and allow us to measure the rotation rate of the bar(s). We measure apattern speed, for both bars, of 1.87 ±0.02 times the local circular frequency. Thisis on par with previous measurements for the Galactic bar, although we do adopt aslightly different Solar motion. Finally, we identify a new kinematic feature in localvelocity space, caused by the long bar, which is tempting to identify with the highvelocity ‘Arcturus’ stream.Key words: Galaxy: centre – Galaxy: kinematics and dynamics – solar neighbour-hood – Galaxy: structure.