The relative efficiencies of bars and clumps in driving disc stars to
  retrograde motion
  The presence of stars on retrograde orbits in disc galaxies is usually
attributed to accretion events, both via direct accretion, as well as through
the heating of the disc stars. Recent studies have shown that retrograde orbits
can also be produced via scattering by dense clumps, which are often present in
the early stages of a galaxy's evolution. However, so far it has been unclear
whether other internally-driven mechanisms, such as bars, are also capable of
driving retrograde motion. Therefore, in this paper, we investigate the
efficiencies with which bars and clumps produce retrograde orbits in disc
galaxies. We do this by comparing the retrograde fractions and the spatial
distributions of the retrograde populations in four $N$-body$+$smooth particle
hydrodynamics (SPH) simulations of isolated disc galaxies spanning a range of
evolutionary behaviours. We find that both bars and clumps are capable of
generating significant retrograde populations of order $\sim 10\%$ of all
stars. We also find that while clump-driven retrograde stars may be found at
large galactocentric radii, bar-driven retrograde stars remain in the vicinity
of the bar, even if the bar dissolves. Consequently, we find that retrograde
stars in the Solar Neighbourhood in the clumpy models are exclusively
clump-driven, but this is a trace population, constituting $0.01-0.04\%$ of the
total stellar population in this region. Finally, we find that neither bars
(including dissolving ones) nor clumps in the models are able to produce
rotationally supported counter-rotating discs.
