Absolute evaporation rates of non-rotating neutral PAH clusters

Clusters of polycyclic aromatic hydrocarbons (PAHs) have been proposed as candidates for evaporating very small grains, which are thought to be precursors of free-flying PAHs. Evaporation rates have been calculated so far only for species containing up to a few 100 C atoms, whereas interstellar PAH clusters could contain up to ~1000 C atoms. We present a method that generalises the calculation of the statistical evaporation rate of large PAH clusters and provides rates for species containing up to ~1000 C-atoms. The evaporation of non-rotating neutral homo-molecular PAH clusters containing up to 12 molecules from a family of highly symmetric compact PAHs is studied. Statistical calculations were performed and completed with molecular dynamics simulations at high internal energies to provide absolute values for the evaporation rate and distributions of kinetic energy released. The calculations used explicit atom-atom Lennard-Jones potentials in the rigid molecule approximation. A new method is proposed to take both inter- and intra-molecular vibrations into account. Without any parameter adjustment, the calculated evaporation rates agree well with available experimental data. We find that the non-rotation assumption has a limited impact on the evaporation rates. The photostability of PAH clusters increases dramatically with the size of molecules in the clusters, and to a lesser extent with the number of molecules in the clusters. For values of the UV radiation field that are typical of the regions where evaporating very small grains are observed, the smallest clusters in this study (~50 C-atoms) are found to be quickly photo-evaporated, whereas the largest clusters (~1000 C-atoms) are photostable. Our results support the idea that large PAH clusters are good candidates for evaporating very small grains.Comment: 13 pages, 10 figure

Detection of the buckminsterfullerene cation (C60+) in space

In the early 90s, C60+ was proposed as the carrier of two diffuse interstellar bands (DIBs) at 957.7 and 963.2 nm, but a firm identification still awaits gas-phase spectroscopic data. Neutral C60, on the other hand, was recently detected through its infrared emission bands in the interstellar medium and evolved stars. In this contribution, we present the detection of C60+ through its infrared vibrational bands in the NGC 7023 nebula, based on spectroscopic observations with the Spitzer space telescope, quantum chemistry calculation, and laboratory data from the literature. This detection supports the idea that C60+ could be a DIB carrier, and provides robust evidence that fullerenes exist in the gas-phase in the interstellar medium. Modeling efforts to design specific observations, combined with new gas-phase data, will be essential to confirm this proposal. A definitive attribution of the 957.7 and 963.2 nm DIBs to C60+ would represent a significant step forward in the field.Comment: To appear in "Proceedings of IAU 297 symposium on the Diffuse Interstellar Bands", eds. J. Cami and N. Cox (5 pages

Evolution of PAHs in photodissociation regions: Hydrogenation and charge states

Various studies have emphasized variations of the charge state and composition of the interstellar polycyclic aromatic hydrocarbon (PAH) population in photodissociation regions (PDRs). We aim to model the spatial evolution of the charge and hydrogenation states of PAHs in PDRs. We focus on the specific case of the north-west (NW) PDR of NGC 7023 and also discuss the case of the diffuse interstellar medium (ISM). The physical conditions in NGC 7023 NW are modelled using a state-of-the-art PDR code. We then use a new PAH chemical evolution model that includes recent experimental data on PAHs and describes multiphoton events. We consider a family of compact PAHs bearing up to 96 carbon atoms. The calculated ionization ratio is in good agreement with observations in NGC 7023 NW. Within the PDR, PAHs evolve into three major populations: medium-sized PAHs (50<Nc<90) are normally hydrogenated, larger PAHs (Nc>90) can be superhydrogenated, and smaller species (Nc<50) are fully dehydrogenated. In the cavity, where the fullerene C60 was recently detected, all the studied PAHs are found to be quickly fully dehydrogenated. PAH chemical evolution exhibits a complex non-linear behaviour as a function of the UV radiation field because of multiphoton events. Steady state for hydrogenation is reached on timescales ranging from less than a year for small PAHs, up to 10000 years for large PAHs at Av=1. We identified critical reactions that need more studies. Our new model allows us to rationalize the observational constraints without any fitting parameter. PAHs smaller than 50 carbon atoms are not expected to survive in the NGC 7023 NW PDR. A similar conclusion is obtained for the diffuse ISM. Carbon clusters turn out to be end products of PAH photodissociation, and the evolution of these clusters needs to be investigated further to evaluate their impact on the chemical and physical evolution of PDRs.Comment: 16 pages, 10 figures; Accepted for publication in A&

Top-down formation of fullerenes in the interstellar medium

[Abridged] Fullerenes have been recently detected in various circumstellar and interstellar environments, raising the question of their formation pathway. It has been proposed that they can form by the photo-chemical processing of large polycyclic aromatic hydrocarbons (PAHs). Following our previous work on the evolution of PAHs in the NGC 7023 reflection nebula, we evaluate, using photochemical modeling, the possibility that the PAH C$_{66}$H$_{20}$ (i.e. circumovalene) can lead to the formation of C$_{60}$ upon irradiation by ultraviolet photons. The chemical pathway involves full dehydrogenation, folding into a floppy closed cage and shrinking of the cage by loss of C$_2$ units until it reaches the symmetric C$_{60}$ molecule. At 10" from the illuminating star and with realistic molecular parameters, the model predicts that 100% of C$_{66}$H$_{20}$ is converted into C$_{60}$ in $\sim$ 10$^5$ years, a timescale comparable to the age of the nebula. Shrinking appears to be the kinetically limiting step of the whole process. Hence, PAHs larger than C$_{66}$H$_{20}$ are unlikely to contribute significantly to the formation of C$_{60}$, while PAHs containing between 60 and 66 C atoms should contribute to the formation of C$_{60}$ with shorter timescales, and PAHs containing less than 60 C atoms will be destroyed. Assuming a classical size distribution for the PAH precursors, our model predicts absolute abundances of C$_{60}$ are up to several $10^{-4}$ of the elemental carbon, i.e. less than a percent of the typical interstellar PAH abundance, which is consistent with observational studies. According to our model, once formed, C$_{60}$ can survive much longer than other fullerenes because of the remarkable stability of the C$_{60}$ molecule at high internal energies.Hence, a natural consequence is that C$_{60}$ is more abundant than other fullerenes in highly irradiated environments.Comment: Accepted for publication in A&A. Latest version contains the corrected version of Fig.

The infrared signatures of very small grains in the Universe seen by JWST

The near- and mid-IR spectrum of many astronomical objects is dominated by emission bands due to UV-excited polycyclic aromatic hydrocarbons (PAH) and evaporating very small grains (eVSG). Previous studies with the ISO, Spitzer and AKARI space telescopes have shown that the spectral variations of these features are directly related to the local physical conditions that induce a photo-chemical evolution of the band carriers. Because of the limited sensitivity and spatial resolution, these studies have focused mainly on galactic star-forming regions. We discuss how the advent of JWST will allow to extend these studies to previously unresolved sources such as near-by galaxies, and how the analysis of the infrared signatures of PAHs and eVSGs can be used to determine their physical conditions and chemical composition.Comment: To appear in the Proceedings of the annual meeting of the French society of astronomy and astrophysics (SF2A 2015

30 years of cosmic fullerenes

In 1985, "During experiments aimed at understanding the mechanisms by which long-chain carbon molecules are formed in interstellar space and circumstellar shells", Harry Kroto and his collaborators serendipitously discovered a new form of carbon: fullerenes. The most emblematic fullerene (i.e. C$_{60}$ "buckminsterfullerene"), contains exactly 60 carbon atoms organized in a cage-like structure similar to a soccer ball. Since their discovery impacted the field of nanotechnologies, Kroto and colleagues received the Nobel prize in 1996. The cage-like structure, common to all fullerene molecules, gives them unique properties, in particular an extraordinary stability. For this reason and since they were discovered in experiments aimed to reproduce conditions in space, fullerenes were sought after by astronomers for over two decades, and it is only recently that they have been firmly identified by spectroscopy, in evolved stars and in the interstellar medium. This identification offers the opportunity to study the molecular physics of fullerenes in the unique physical conditions provided by space, and to make the link with other large carbonaceous molecules thought to be present in space : polycyclic aromatic hydrocarbons.Comment: To appear in the Proceedings of the annual meeting of the French society of astronomy and astrophysics (SF2A 2015

Gas morphology and energetics at the surface of PDRs: New insights with Herschel observations of NGC 7023

Context. We investigate the physics and chemistry of the gas and dust in dense photon-dominated regions (PDRs), along with their dependence on the illuminating UV field. Aims. Using Herschel/HIFI observations, we study the gas energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023 is the prototype of a PDR illuminated by a B2V star and is one of the key targets of Herschel. Methods. Our approach consists in determining the energetics of the region by combining the information carried by the mid-IR spectrum (extinction by classical grains, emission from very small dust particles) with that of the main gas coolant lines. In this letter, we discuss more specifically the intensity and line profile of the 158 μm (1901 GHz) [C ii] line measured by HIFI and provide information on the emitting gas. Results. We show that both the [C ii] emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) arise from the regions located in the transition zone between atomic and molecular gas. Using the Meudon PDR code and a simple transfer model, we find good agreement between the calculated and observed [C ii] intensities. Conclusions. HIFI observations of NGC 7023 provide the opportunity to constrain the energetics at the surface of PDRs. Future work will include analysis of the main coolant line [O i] and use of a new PDR model that includes PAH-related species