Dust Destruction by Charging: A Possible Origin of Grey Extinction
  Curves of Active Galactic Nuclei
  Observed extinction curves of active galactic nuclei (AGNs) are significantly
different from those observed in the Milky Way. The observations require
preferential removal of small grains at the AGN environment; however, the
physics for this remains unclear. In this paper, we propose that dust
destruction by charging, or Coulomb explosion, may be responsible for AGN
extinction curves. Harsh AGN radiation makes a dust grain highly charged
through photoelectric emission, and grain fission via the Coulomb explosion
occurs when the electrostatic tensile stress of a charge grain exceeds its
tensile strength. We show that the Coulomb explosion can preferentially remove
both small silicate and graphite grains and successfully reproduce both flat
extinction curves and the absence of 2175\AA~bump.
