The impact of astrophysical dust grains on the confinement of cosmic
  rays
  We argue that charged dust grains could significantly impact the confinement
and transport of galactic cosmic rays. For sub-GeV to ~1000GeV cosmic rays,
small-scale parallel Alfv\'en waves, which isotropize cosmic rays through
gyro-resonant interactions, are also gyro-resonant with charged grains. If the
dust is nearly stationary, as in the bulk of the interstellar medium, Alfv\'en
waves are damped by dust. This will reduce the amplitude of Alfv\'en waves
produced by the cosmic rays through the streaming instability, thus enhancing
cosmic-ray transport. In well-ionized regions, the dust damping rate is larger
by a factor of ~10 than other mechanisms that damp parallel Alfv\'en waves at
the scales relevant for ~GeV cosmic rays, suggesting that dust could play a key
role in regulating cosmic-ray transport. In astrophysical situations in which
the dust moves through the gas with super-Alfv\'enic velocities, Alfv\'en waves
are rendered unstable, which could directly scatter cosmic rays. This
interaction has the potential to create a strong feedback mechanism where dust,
driven through the gas by radiation pressure, then strongly enhances the
confinement of cosmic rays, increasing their capacity to drive outflows. This
mechanism may act in the circumgalactic medium around star-forming galaxies and
active galactic nuclei.
