Modelling Grain Rotational Disruption by Radiative Torques and
  Extinction of Active Galactic Nuclei
  Extinction curves observed toward individual Active Galactic Nuclei (AGN)
usually show a steep rise toward far-ultraviolet (FUV) wavelengths and can be
described by the Small Magellanic Cloud (SMC)-like dust model. This feature
suggests the dominance of small dust grains of size $a \leq 0.1 \mu m$ in the
local environment of AGN, but the origin of such small grains is unclear. In
this paper, we aim to explain this observed feature by applying the RAdiative
Torque Disruption (RATD) to model the extinction of AGN radiation from FUV to
mid-infrared (MIR) wavelengths. We find that in the intense radiation field of
AGN, large composite grains of size $a \geq 0.1 \mu m$ are significantly
disrupted to smaller sizes by RATD up to $d_{\rm RATD} > 100$ pc in the polar
direction and $d_{\rm RATD} \sim 10$ pc in the torus region. Consequently,
optical-MIR extinction decreases, whereas FUV-near-ultraviolet (NUV) extinction
increases, producing a steep far-UV rise extinction curve. The resulting
total-to-selective visual extinction ratio thus significantly drops to $R_{\rm
V} < 3.1$ with decreasing distances to AGN center due to the enhancement of
small grains. The dependence of $R_{\rm V}$ with the efficiency of RATD will
help us to study the dust properties in the AGN environment via photometric
observations. In addition, we suggest that the combination of the strength
between RATD and other dust destruction mechanisms which are responsible for
destroying very small grains of $a \leq 0.05 \mu m$ is the key for explaining
the dichotomy observed 'SMC' and 'gray' extinction curve toward many AGN.
