What can we learn about protoplanetary disks from analysis of mid-infrared carbonaceous dust emission?

Context. The disks of gas and dust that form around young stars and can lead to planet formation contain polycyclic aromatic hydrocarbons (PAHs) and very small grains (VSGs). Aims. In this paper we analyze the mid-infrared (mid-IR) emission of these very small dust particles in a sample of 12 protoplanetary disks. Our goal is twofold: first we want to characterize the properties of these particles in disks and see how they are connected to interstellar matter, and second we investigate the possibility that their emission can be used as a probe of the physical conditions and evolution of the disk. Methods. We define a basis made of three mid-IR template spectra: PAH(0), PAH(+), and VSGs that were derived from the analysis of reflection nebulae, and an additional PAH(x) spectrum that was introduced recently for analysis of the spectra of planetary nebulae. Results. From the optimization of the fit of 12 star+disk spectra, using a linear combination of the 4 template spectra, we found that an additional small grain component with a broad feature at 8.3 mu m is needed. We find that the fraction of VSG emission in disks decreases with increasing stellar temperature. VSGs appear to be destroyed by UV photons at the surface of disks, thus releasing free PAH molecules, which are eventually ionized as observed in photodissociation regions. In contrast, we observe that the fraction of PAH(x) increases with increasing star temperature except in the case of B stars where they are absent. We argue that this is compatible with the identification of PAH(x) as large ionized PAHs, most likely emitting in regions of the disk that are close to the star. Finally, we provide a UV-dependent scheme to explain the evolution of PAHs and VSGs in protoplanetary disks. These results allow us to put new constraints on the properties of two sources: IRS 48 and Gomez's Hamburger which are poorly characterized. Conclusions. Very small dust particles incorporated into protoplanetary disks are processed while exposed to the intense radiation field of the central star. The resulting shape of the mid-IR spectrum can reveal this processing and be used as an efficient probe of the radiation field i.e. luminosity of central star.