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The infrared signature of polycyclic aromatic hydrocarbons (PAHs) are present in many protostellar disks and these speciesare thought to play an important role in heating of the gas in the photosphere. We aim to consider PAH cluster formation as one possible cause for non-detections of PAH features in protoplanetary disks. We test the necessary conditions for cluster formation and cluster dissociation by stellar optical and FUV photons in protoplanetarydisks using a Herbig Ae/Be and a T Tauri star disk model. We perform Monte-Carlo (MC) and statistical calculations to determine dissociation rates for coronene, circumcoronene and circumcoronene clusters with sizes between 2 and 200 cluster members. By applying general disk models to our Herbig Ae/Be and T Tauri star model, we estimate the formation rate of PAH dimers and compare these with the dissociation rates. We show that the formation of PAH dimers can take place in the inner 100 AU of protoplanetary disks in sub-photospheric layers. Dimer formation takes seconds to years allowing them to grow beyond dimer size in a short time. We further demonstrate that PAH cluster increase their stability while they grow if they are located beyond a critical distance that depends on stellar properties and PAH species. The comparison with the local vertical mixing time scale allows a determination of the minimum cluster size necessaryfor survival of PAH clusters. Considering the PAH cluster formation sites, cluster survival in the photosphere of the inner disk of Herbig stars isunlikely because of the high UV radiation. For the T Tauri stars, survival of coronene, circumcoronene and circumcircumcoronene clusters is possible and cluster formation should be considered as one possible explanation for low PAH detection rates in T Tauri star disks.
Context: PAHs appear to be an ubiquitous interstellar dust component but the effects of shocks waves upon them have never been fully investigated. Aims: To study the effects of energetic (~0.01-1 keV) ion (H, He and C) and electron collisions on PAHs
Context: PAHs are thought to be a ubiquitous and important dust component of the interstellar medium. However, the effects of their immersion in a hot (post-shock) gas have never before been fully investigated. Aims: We study the effects of energetic
We have examined polycyclic aromatic hydrocarbon (PAH) excitation in a sample of 25 nearby face-on spiral galaxies using the ratio of mid-infrared PAH emission to dust mass. Within 11 of the galaxies, we found that the PAH excitation was straightforw
Context: Cosmic rays are present in almost all phases of the ISM. PAHs and cosmic rays represent an abundant and ubiquitous component of the interstellar medium. However, the interaction between them has never before been fully investigated. Aims: To
We examine polycyclic aromatic hydrocarbon (PAH), dust and atomic/molecular emission toward the Galactic bulge using Spitzer Space Telescope observations of four fields: C32, C35, OGLE and NGC 6522. These fields are approximately centered on (l, b) =