All Figures

	Figure 1: Absorption coefficient (in cm2 per gram of gas) of large grains (BGs; dotted line), very small grains (VSGs; dashed line) and PAHs of 100 C atoms (solid line). See text for details.
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In the text

Figure 2: Disk properties for the template model. The lower panel shows the run with radius of the pressure scale height , the photospheric scale height and the flare angle  divided by 20 deg for convenience. is 1.4 deg at the inner radius, and reaches 7 deg at the outer disk radius. The top panel shows the temperature for graphite (solid line) and silicate (dashed line) grains of size 0.01 m at .

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In the text

Figure 3: Spectral energy distribution of a star/disk system (template model) seen face-on at a distance of 150 pc. The solid line shows the emission of the disk, the dotted line that of the stellar photosphere. The dashed line shows the SED of a model which has all the parameters of the template model but no VSGs and no PAHs. The stellar parameters are  = 10 500 K,  = 32 ,  = 2.4 . The disk is flared, it has a mass of 0.1 , an inner radius of 0.3 AU and an outer radius of 300 AU (see Fig. 2).

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In the text

	Figure 4: Cumulative intensity of the feature (continuum subtracted, solid lines) and continuum (dotted lines) at 3.3, 6.2 and 11.3 m as function of the projected radius. We plot for comparison the run with radius of the cumulative fraction of the FUV radiation intercepted by the disk surface (dashed lines).
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In the text

Figure 5: Surface brightness profiles convolved with a two-dimensional gaussian of FWHM = 0.1 at the peak of the feature (no continuum subtracted, solid lines) and in the adjacent continuum (dotted lines) for the 3.3, 6.2 and 11.3 m features. In the top panel, we also show the photospheric stellar emission at 3.3 m convolved with the same beam (dashed line). As in Fig. 3, the star/disk is seen face-on at a distance of 150 pc.

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In the text

Figure 6: The top panel shows the SEDs of models containing positively ionized PAH (solid thick line) and partially dehydrogenated PAHs with (dotted line). All the other parameters are as in the template model. For comparison, we show again also the SED of the template model (solid thin line). Note that the fluxes of the ionized PAH model and of the template model have been multiplied by factors 10 and 100, respectively. The bottom panel plots the brightness profiles convolved as in Fig. 5 at the peak of the 3.3, 6.2 and 11.3 m features (no continuum subtracted), as labelled. The lines refer to the ionized PAH model (solid thick), the dehydrogenated PAH model (dotted) and to the template model (solid thin).

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In the text

	Figure 7: Same as Fig. 6 for model containing neutral PAHs with (solid thick lines) and ionized PAHs with (dotted lines). Again, we show also the template model by solid thin lines.
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In the text

	Figure 8: Strength of the PAH features at 3.3 (solid line), 6.2 (dotted line) and 11.3 m (dashed line) for ZAMS stars. The scale on the upper axis shows the spectral type, that on the lower axis the corresponding value of . The thin solid line shows a linear dependence . Disk and dust parameters are as in the template model, which has ( = 32  and  K).
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In the text

Figure 9: Comparison of model predictions and observations. Upper Panel: Strength of the 3.3 m feature normalised to the FUV flux of the star for ZAMS stars. As in Fig. 8, the scale on the upper axis is the spectral type, that on the lower axis is the value of , i.e., the FUV field at 150 AU from the star. The two solid lines show the predictions of the template model for (lower curve) and (upper curve) (see text). The two thin lines show an uncertainty strip of  a factor of 2 around these models. The crosses show observed intensities (normalized to the corresponding FUV flux) for the isolated HAeBe stars with evidence of a flared disk, the diamonds for the isolated HAeBe stars with no evidence of a flared disk and the triangles for the intermediate sources (HD 97048 and Elias 1). The asterisks show the HAeBe stars observed from the ground by Brooke et al. (1993); the square is WL 16, corrected for dust attenuation (see text). Error bars, when not shown, are smaller than the symbols. Arrows are 5 upper limits. Medium and lower Panels: Same as upper panel for the 6.2 and 11.3 m feature.

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In the text