Hot graphite dust in the inner regime of NGC 4151
  We model the near infrared SED of NGC 4151 with a 3-D radiative transfer
SKIRT code, using which torus only (TO) and Ring And Torus (RAT) scenarios are
studied. In the RAT models, a graphite ring-like structure (clumpy or smooth),
is incorporated between the torus and the accretion disk. We vary the
inclination angle $(i)$, inner radius (of the torus and the ring, $R_{\rm in,
t}$ and $R_{\rm in, r}$ respectively), torus half-opening angle ($\sigma $),
optical depth ($\tau_{9.7, \rm t} $ of the torus and $\tau_{9.7, \rm r} $ of
the ring ) and the dust clump size ($R_{\rm clump}$). We perform a statistical
analysis of the parameter space and find that all the models are able to
explain the flat NIR SED of NGC 4151 with minor differences in the derived
parameters. For the TO model, we get, $R_{\rm in, t}=0.1$ pc, $\sigma =
30^\circ $, $i = 53^\circ $, $\tau_{9.7, \rm t}=10$ and the clumpsize, $R_{\rm
clump}$ =0.4 pc. For the smooth RAT model, $R_{\rm in, \rm r}=0.04$ pc,
$\tau_{9.7, \rm total}$ = 11 and for the clumpy RAT model, $R_{\rm in, r} =
0.04$ pc/0.06 pc and $\tau_{9.7, \rm total}=20$. The $R_{\rm in, t}$ from the
TO model does not agree with the NIR observations ($\sim 0.04$ pc). Hence, the
most likely scenario is that a hot graphite ring is located at a distance 0.04
pc from the centre, composed of a smooth distribution of dust followed by a
dusty torus at 0.1 pc with ISM type of grains.
