The dusty tori of nearby QSOs as constrained by high-resolution mid-IR
  observations
  We present mid-infrared (MIR, 7.5-13.5 $\mu$m) imaging and spectroscopy
observations obtained with the CanariCam (CC) instrument on the 10.4m Gran
Telescopio CANARIAS for a sample of 20 nearby, MIR bright and X-ray luminous
QSOs. We find that for the majority of QSOs the MIR emission is unresolved at
angular scales nearly 0.3 arcsec, corresponding to physical scales $<600$ pc.
We find that the higher-spatial resolution CC spectra have similar shapes to
those obtained with Spitzer/IRS, and hence we can assume that the spectra are
not heavily contaminated by extended emission in the host galaxy. We thus take
advantage of the higher signal to noise Spitzer/IRS spectra, as a fair
representation of the nuclear emission, to decompose it into a combination of
active galactic nuclei (AGN), polycyclic aromatic hydrocarbon (PAH) and stellar
components. In most cases the AGN is the dominant component, with a median
contribution of 85 per cent of the continuum light at MIR (5-15 $\mu$m) within
the IRS slit. This IR AGN emission is well reproduced by clumpy torus models.
We find evidence for significant differences in the parameters that describe
the dusty tori of QSOs when compared with the same parameters of Seyfert 1 and
2 nuclei. In particular, we find a lower number of clouds ($N_{0}<12$), steeper
radial distribution of clouds ($q=1.5-3.0$), and clouds that are less optically
thick ($\tau_{V}<100$) than in Seyfert 1, which could be attributed to dusty
structures that have been partially evaporated and piled up by the higher
radiation field in QSOs. We find that the combination of the angular width
$\sigma_{torus}$, viewing angle $i$, and number of clouds along the equatorial
line $N_{0}$, produces large escape probabilities ($P_{esc} > 2$ per cent) and
low geometrical covering factors ($f_{2}<0.6$), as expected for AGN with broad
lines in their optical spectra.
