Context. This work was conducted as part of the SPECPDR program, dedicated to the study of very small particles and astrochemistry, in Photo-Dissociation Regions (PDRs). Aims. We present the analysis of the mid-IR spectro-imagery observations of Ced 201, NCG 7023 East and North-West and ρ Ophiuchi West filament. Methods. Using the data from all four modules of the InfraRed Spectrograph onboard the Spitzer Space Telescope, we produced a spectral cube ranging from 5 to 35 µm, for each one of the observed PDRs. The resulting cubes were analysed using Blind Signal Separation methods (NMF and FastICA). Results. For Ced 201, ρ Ophiuchi West filament and NGC 7023 East, we find that two signals can be extracted from the original data cubes, which are 5 to 35 µm spectra. The main features of the first spectrum are a strong continuum emission at long wavelengths, and a broad 7.8 µm band. On the contrary, the second spectrum exhibits the classical Aromatic Infrared Bands (AIBs) and no continuum. The reconstructed spatial distribution maps show that the latter spectrum is mainly present at the cloud surface, close to the star whereas the first one is located slightly deeper inside the PDR. The study of the spectral energy distribution of Ced 201 up to 100 µm suggests that, in cool PDRs, the 5-25 µm continuum is carried by Very Small Grains (VSGs). The AIB spectra in the observed objects can be interpreted as the contribution of neutral and positively-charged Polycyclic Aromatic Hydrocarbons (PAHs). Conclusions. We extracted the 5 to 25 µm emission spectrum of VSGs in cool PDRs, these grains being most likely carbonaceous. We show that the variations of the mid-IR (5-35 µm) spectra of PDRs can be explained by the photo-chemical processing of these VSGs and PAHs, VSGs being the progenitors of free PAHs.

Buckminsterfullerene (C 60 ) was recently confirmed as the largest molecule identified in space. However, it remains unclear how and where this molecule is formed. It is generally believed that C 60 is formed from the buildup of small carbonaceous compounds in the hot and dense envelopes of evolved stars. Analyzing infrared observations, obtained by Spitzer and Herschel, we found that C 60 is efficiently formed in the tenuous and cold environment of an interstellar cloud illuminated by strong ultraviolet (UV) radiation fields. This implies that another formation pathway, efficient at low densities, must exist. Based on recent laboratory and theoretical studies, we argue that polycyclic aromatic hydrocarbons are converted into graphene, and subsequently C 60 , under UV irradiation from massive stars. This shows that alternative-top-downroutes are key to understanding the organic inventory in space.fullerene | interstellar medium T he midinfrared spectra of a variety of astrophysical objects are dominated by band emission (strongest at 3.3, 6.2, 7.7, 8.6, and 11.2 μm) attributed to carbonaceous macromolecules [i.e., polycyclic aromatic hydrocarbons (PAHs)] (1). These molecules are large (30-100 C atoms), abundant (approximately 5% of the elemental carbon), and their ionization plays a key role in the energy balance of gas in the interstellar medium (ISM) and in protoplanetary disk. In addition to PAH bands, infrared signatures observed at 7.0, 8.5, 17.4, and 19.0 μm have been reported recently (2, 3) and found to coincide precisely with the emission of buckminsterfullerene (C 60 ) (4), a cage-like carbon molecule. This detection heralds the presence of a rich organic inventory and chemistry in space. However, observed abundances of C 60 challenge the standard ion-molecule or grain-surface chemistry formation routes, which build up molecules from small to large in the ISM. For that reason, it has been suggested that C 60 is formed in the hot and dense envelopes of evolved stars (5-7) in processes similar to those found in sooty environments (8-11), and eventually, is ejected in space. Yet, this scenario faces the problem that it has a limited efficiency (6). PAHs and C 60 are known to coexist in the ISM (3); however, so far, the connection between PAHs and C 60 -and in particular the possibility to go from one compound to the other in space-has not been investigated. In this paper, we present a study of PAH and C 60 chemical evolution in the NGC 7023 nebula, using Spitzer (12) and Herschel (13) infrared observations. Observational ResultsInfrared Observations of the NGC 7023 Nebula. Earlier Spitzer observations of the NGC 7023 reflection nebula have revealed a chemical evolution of PAHs: Deep in the cloud, emission is dominated by PAH clusters, which evaporate into free-flying PAHs when exposed to the UV radiation from the star (14-16). There, gaseous PAHs are, in turn, ionized. While the neutral PAHs are dominated by zig-zag edges-as demonstrated by the strong C-H solo out-of-plane modes-the ions have an armchair ...

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