Metallicity dependence of the Hercules stream in Gaia/RAVE data --
  explanation by non-closed orbits
  The origin of the Hercules stream, the most prominent velocity substructure
in the Solar neighbour disc stars, is still under debate. Recent accurate
measurements of position, velocity, and metallicity provided by Tycho Gaia
Astrometric Solution (TGAS) and RAdial Velocity Experiments (RAVE) have
revealed that the Hercules stream is most clearly seen in the metal-rich region
([Fe/H] > 0), while it is not clearly seen in lower metallicity region ([Fe/H]
< -0.25). By using a large number of chemo-dynamical 2D test-particle
simulations with a rotating bar and/or spiral arms, we find that the observed
[Fe/H] dependence of the Hercules stream is a natural consequence of the
inside-out formation of the stellar disc and the existence of highly non-closed
orbits in the rotating frame of the bar or spiral arms. Our successful models
that reproduce the observed properties of the Hercules stream include not only
fast-bar-only and fast-bar+spiral models, but also slow-bar+spiral models. This
indicates that it is very difficult to estimate the pattern speed of the bar or
spiral arms based only on the observations of the Hercules stream in the Solar
neighbourhood. As a by-product of our simulations, we make some predictions
about the locations across the Galactic plane where we can observe velocity
bimodality that is not associated with the Hercules stream. These predictions
can be tested by the Gaia Data Release 2, and such a test will improve our
understanding of the evolution of the Milky Way stellar disc.
