tokens: (e.g. Desert et al. 1990 ; Draine Li 2001 ; Compiègne et al. 2011 ). The grain structure and optical properties were therefore calculated by considering their composition and size, and the resulting spectra are directly relationed to these properties. In order to facilitate comparison with previous models and studies, we distinguish three sub-populations of a-C grains: a-C1: a-C nanoparticles with a ≤ 0.7 nm, which would be equivalent to the smallest PAH grains in Desert et al. (1990) , Draine Li (2001) and Compiègne et al. (2011) and represent ~10% of the mass for all models; a-C2: a-C with 0.7 a ≤ 1.5 nm, which would be equivalent to the largest PAHs in Desert et al. (1990) , Draine Li (2001) and Compiègne et al. (2011) ; very small grains (VSG): larger a-C particles with a > 1.5 nm, which would be equivalent to the VSG as defined in Desert et al. (1990) , the smallest carbon grains with graphite optical properties in Draine Li (2001) , and the small amorphous carbon grains (SamC) in Compiègne et al. (2011) . Fig. 5 Upper panel : measured FWHM in VISIR long-slit spectra. The ozone atmospheric absorption band is delimited by the grey shaded area. The positions of aromatic (blue) and silicate crystalline (red) main features are shown as dashed lines. The estimated 3 σ error on the FWHM is 2.5 10 −3′′ . The purple points are the measures for HD 100546. The same reference star (HD 91056, green points) as was used for photometriccalibration was used and is assumed to reflect the angular resolution response of the atmosphere combined to the telescope or instrument. Bottom panel : ratio between HD 100546 and standard star FWHM shown in the upper panel. The raw ratio obtained by crude division of the FWHMs was smoothed it with a three-pixel-wide boxcar having the spectral width of the actual spectral sampling in order to reduce pixel-to-pixel irregularities. The estimated 3 σ error on the ratio is about 1.2% (~0.02). Fig. 6 Top panel : 8.6 μm (brown) and 11.3 μm (magenta) aromatic emission bands and 10.4 μm (black) continuum emission plotted according to the distance to the star. The 1∕ d 2 function is also plotted (black dotted). Bottom panel : 8.6 μm/10.4 μm and 11.3 μm/10.4 μm band ratios plotted according the distance to the star. The transparent grey box on the left represents the distance up to which the cavity is extended in POLARIS simulations, ~ 0.12′′ (13 au). At large distances, the variations in the band-to-continuum ratios are not significant. The error bars on the band-to-continuum ratio are about 5%. The dust mass fraction of each THEMIS dust population and a-C sub-population, that is, the population dust mass M X to the total dust mass M total ratio, q X = M X ∕ M total are, q a-C1 ~ 10%, q a-C2 ~ 7%, q VSG ~ 6%, q a-C:H/a-C ~ 8% and q a-Sil/a-C ~ 69%. Finally, for simplicity, we considered only amorphous silicate grains in our model, while several signatures from crystalline silicates have clearly been observed towards HD
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