Dust Destruction by Drift-Induced Sputtering in Active Galactic Nuclei
  Recent mid-infrared high spatial resolution observations have revealed that
active galactic nuclei (AGNs) may host a polar dust region with the size of
several pc, and such dust may be carried by radiation from the central engine.
The polar dust emission often exhibits very weak or absence of the silicate
10-$\mu$m emission feature. A possible explanation is that the polar dust is
dominated by micron-sized large grains because these grains do not show the
silicate feature, while it remains unclear how large grains are preferentially
supplied to the polar region. We here propose a new scenario describing the
prevalence of large grains at the polar region. We show that grains are
accelerated to the hypersonic drift velocity by the radiation pressure from
AGN, and the hypersonic drift results in dust destruction via kinetic
sputtering. Sputtering destroys small grains faster than the large ones, and
thus larger grains will be preferentially blown over longer distance. Although
the hypersonic drift, or kinetic sputtering, tends to be suppressed for very
small grains due to the Coulomb drag, they might also be disrupted by Coulomb
explosion. Removal of small grains and/or survival of large grains may explain
the lack of a silicate 10-$\mu$m emission feature in polar dust emission.
