Context. We have modelled the abundance distribution and IR emission of the first 3 members of the coronene family in the north-west photodissociation region of the well-studied reflection nebulae NGC 7023. Aims. Our aim was 3-fold: i) analyze the distribution of abundances; (ii) examine the spectral footprints from the hydrogenation state of polycyclic aromatic hydrocarbons (PAHs); and (iii) assess the role of PAHs in the formation of H 2 in photodissociation regions. Methods. To model the physical conditions inside the cloud, we used the Meudon PDR Code, and we gave this as input to our kinetic model. We used specific molecular properties for each PAH, based on the latest data available at the present time. We considered the loss of an H atom or an H 2 molecule as multiphoton processes, and we worked under the premise that PAHs with extra H atoms can form H 2 through an Eley-Rideal abstraction mechanism. Results. In terms of abundances, we can distinguish clear differences with PAH size. The smallest PAH, coronene (C 24 H 12 ), is found to be easily destroyed down to the complete loss of all of its H atoms. The largest species circumcircumcoronene (C 96 H 24 ), is found in its normal hydrogenated state. The intermediate size molecule, circumcoronene (C 54 H 18 ), shows an intermediate behaviour with respect to the other two, where partial dehydrogenation is observed inside the cloud. Regarding spectral variations, we find that the emission spectra in NGC 7023 are dominated by the variation in the ionization of the dominant hydrogenation state of each species at each point inside the cloud. It is difficult to "catch" the effect of dehydrogenation in the emitted PAH spectra since, for any conditions, only PAHs within a narrow size range will be susceptible to dehydrogenation, being quickly stripped off of all H atoms (and may isomerize to cages or fullerenes). The 3 µm region is the most sensitive one towards the hydrogenation level of PAHs. Conclusions. Based on our results, we conclude that PAHs with extra H atoms are not the carriers of the 3.4 µm band observed in NGC 7023, since these species are only found in very benign environments. Finally, concerning the role of PAHs in the formation of H 2 in photodissociation regions, we find that H 2 abstraction from PAHs with extra H atoms is an inefficient process compared to grains. Instead, we propose that photodissociation of PAHs of small-to-intermediate sizes could contribute to H 2 formation in PDR surfaces, but they cannot account by themselves for the inferred high H 2 formation rates in these regions.

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Context. IC 63 and IC 59 are a pair of cometary-shaped nebulae in the vicinity of the star γCas (also known as Tsih, "the Whip"). Both nebulae have very different optical appearances, despite the fact that both objects lie at similar projected distances from the star: IC 63 shows bright rims and filaments, while IC 59 looks more homogeneous and faint. Aims. We aim to perform a general description of the two nebulae from an observational standpoint in order to study the physical conditions at the UV-illuminated surfaces of these objects. Methods. We used the available data on both nebulae taken with Spitzer and Herschel to study the infrared emission at the tip of both clouds, and derive the intensity of the UV radiation field. Using the photodetector array camera and spectrometer (PACS) in line spectroscopy mode, we obtained the intensity of the cooling lines [C II] 157 µm and [O I] 63 µm, and we used these to estimate the density in these regions. Excitation diagrams of molecular hydrogen were obtained to derive the gas temperature. We also used [C II] 157 µm velocity maps of IC 59 taken with German REceiver for Astronomy at Terahertz frequencies (GREAT) on board SOFIA to explore the gas dynamics in this nebula. Results. We find that the IR emission from polycyclic aromatic hydrocarbons (PAHs) is very similar at the tip of both nebulae. Even though it varies in intensity between the two, the derived PAH band ratios are remarkably similar. These ratios are similar to those found in the more shielded regions of other nebulae such as NGC 7023 and NGC 2023. Regarding the physical conditions, we get that while in IC 63 the intensity of the UV field, G 0 , is a factor of approximately ten higher than in IC 59, the density n at the tip of IC 59 is lower than in IC 63 by a similar factor. For both objects we derive G 0 values significantly lower than what previous works have so far assumed. Comparison with other reflection nebulae photo-dissociation regions (PDRs) and known correlations support our claim that both IC 63 and IC 59 are low-UV irradiated environments. Conclusions. We conclude that the tips of IC 63 and IC 59 are about three and five times farther away from the star than their respective projected distances. The similarity of the mid-infrared emission between the two nebulae is consistent not only with both objects being overdensities within the same region around γCas, but it is also consistent with the similar G 0 /n and ionization parameters, which altogether rule the evolution of the hydrogenation and ionization level of the emitting population of PAHs. Finally, regarding the kinematics of the material in IC 59, we find evidence of photo-evaporation due to the incident radiation from γCas.

We present near and mid-IR observations of a sample of Seyfert II galaxies drawn from the 12µm Galaxy Sample. The sample was observed in the J, H, K, L, M and N bands. Galaxy Surface Brightness Profiles are modeled using nuclear, bulge, bar (when necessary) and disk components.To check the reliability of our findings the procedure was tested using Spitzer observations of M 31. Nuclear Spectral Energy Distributions (SEDs) are determined for 34 objects, and optical spectra are presented for 38, including analysis of their stellar populations using the STARLIGHT spectral synthesis code. Emission line diagnostic-diagrams are used to discriminate between genuine AGN and HII nuclei. Combining our observations with those found in the literature, we have a total of 40 SEDs. It is found that about 40% of the SEDs are characterized by an upturn in the near-IR, which we have quantified as a NIR slope α < 1 for an SED characterized as λf λ ∝ λ α . Three objects with an HII nucleus and two Seyfert nuclei with strong contamination from a circumnuclear starburst, also show an upturn. For genuine AGN this component could be explained as emission from the accretion disk, a jet, or from a very hot dust component leaking from the central region through a clumpy obscuring structure. The presence of a very compact nuclear starburst as the origin for this NIR excess emission is not favored by our spectroscopic data for these objects. Subject headings:

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