The mechanisms of nucleation and growth of carbon dust particles in circumstellarenvelopes of carbon-rich stars in the red giant and AGB phases of their evolution arepoorly understood. It has been proposed that the transition of gas phase species to solidparticles,is achieved by the formation of a critical nucleus composed of two PAHs heldtogether by van der Waals forces. Some insights into the validity of the nucleation of PAHmolecules in the envelope can be gained through the investigation of the thermodynamics ofdimers,representing the first stage towards condensation. We have performed experimentsto identify the temperature range over which small PAH clusters form in saturated uniformsupersonic flows. The kinetics of the formation has also been investigated. Theexperimental data have been combined with theoretical calculations. We unambiguouslydemonstrate that the association of small PAHs such as pyrene(C16H10) is slower than the destruction of the dimer in warm andhot environments such as IRC +10216. Our findings challenge a formation model based on thephysical stacking of small PAH units in circumstellar shells of carbon rich stars.

Studies of dust analogs formed from hydrocarbon(CH4, C2H2, C2H4, C2H6) and PAH precursors havebeen performed using a new facility that we have developed to simulate interstellar andcircumstellar processes. The species formed in a plasma are detected,characterized andmonitored in situ with high-sensitivity techniques,which provide bothspectroscopic and ion mass information. From these measurements we derive information onthe nature,the size and the structure of dust particles,as well as a betterunderstanding of the growth and destruction processes of extraterrestrial dust.

The fullerene C60 has four infrared active vibrational transitions at 7.0, 8.5,17.5 and 18.9 μm. We have previously observed emission features at17.4 and 18.9 μm in the reflection nebula NGC 7023 and demonstratedspatial correlations suggestive of a common origin. We now confirm the identification ofthese features with C60 by detecting a third emission feature at7.04 ± 0.05μm at a position of strong 18.9 μm emissionin NGC 7023. We also report the detection of these three features in the reflection nebulaNGC 2023. We show with spectroscopic mapping of NGC 7023 that the 18.9 μmfeature peaks on the central star,that the 16.4 μm emission feature dueto PAHs peaks between the star and a nearby photodissociation front,and that the 17.4μm feature is a blend of a PAH feature and C60. The derivedC60 abundance is consistent with that from previous upper limits and possiblefullerene detections in the interstellar medium.

We present the results of two studies investigating the abundance and physical state ofpolycyclic aromatic hydrocarbons (PAHs) in the Small Magellanic Cloud (SMC). Observationswith ISO and Spitzer have shown that PAHs are deficient in low-metallicity galaxies. Inparticular,galaxies with 12 + log(O/H)  <  8 show mid-IR SEDs and spectraconsistent with low PAH abundance. The SMC provides a unique opportunity to map the PAHemission in a low-metallicity (12  +  log(O/H)  ~ 8) galaxy at high spatialresolution and sensitivity to learn about the PAH life cycle. Using mid- and far-IRphotometry from the Spitzer Survey of the SMC (S3MC) and mid-IR spectralmapping from the Spitzer Spectroscopic Survey of the SMC (S4MC) we determinethe PAH abundance across the galaxy. We find that the SMC PAH abundance is low compared tothe Milky Way and variable,with high abundance in molecular regions and low abundance inthe diffuse ISM. From the variations of the mid-IR band strengths,we show that PAHs inthe SMC are smaller and more neutral than their counterparts in more metal-rich galaxies.Based on the results of these two studies we propose that PAHs in the SMC are formed witha size distribution shifted towards smaller grains and are therefore easier to destroyunder typical diffuse ISM conditions. The distribution of PAH abundance in the SMCsuggests that PAH formation in molecular clouds is an important process. We discuss theimplications of these results for our understanding of the PAH life-cycle both atlow-metallicity and in the Milky Way.

The analysis of mid-IR emission suggests that a population of PAH-related very smallgrains containing a few hundreds of atoms are present in the deep regions of molecularclouds,although no specific species has been identified yet. In this review,we discussseveral candidates for these grains: neutral and ionised PAH clusters and complexes ofPAHs with Si atoms. The theoretical modelling of the properties of such molecularcomplexes or nanograins is a challenging task. We first present an overview of quantumchemistry derived models which can be efficiently used on-the-fly in extensive sampling ofthe potential energy surfaces,as required by structural optimization,classical moleculardynamics or Monte Carlo algorithms. From the simulations,various observables can bedetermined,such as the binding energies,finite temperature IR spectra,nucleation andevaporation rates. We discuss the relevance of those candidates in the molecular cloudsphotodissociation regions and propose constrains and perspectives for the nature and sizeof those very small grains.

PAHs are traditionally thought to form around carbon-rich AGB stars,where the presenceof inner-wind shocks and nucleation seeds can aid their formation. However,my recent workhas shown that the formation of benzene – thought to be the rate-limiting step in theformation of larger PAHs – can be efficient in other environments,namely the dense toriaround the evolved stars of pre-planetary nebulae and in the inner regions of theaccretion discs around young stars. I will discuss the chemical pathways to the formationof benzene in these regions and the subsequent formation of larger PAHs.

We have investigated the dissociative recombination (DR) of theC6D6+and C6D7+ions using the CRYRING heavy-ion storage ring at Stockholm University,Sweden. Thedissociative recombination branching ratios were determined at minimal collision energy, showing that the DR of both ions was dominated by pathways keeping the carbon atomstogether in one product. The absolute reaction cross sections for the titular ions arebest fitted by σ(Ecm [eV]) = 1.3 ± 0.4 × 10-15(Ecm [eV])−1.19 ± 0.02 cm2(C6D6+)and σ(Ecm [eV]) = 1.1 ± 0.3 × 10-15(Ecm [eV])−1.33 ± 0.02 cm2(C6D7+)in the intervals 3-300 meV and 3-200 meV respectively. The thermal rate constants of thetitular reactions are best described by:k(T) = 1.3 ± 0.4 × 10-6(T/300)−0.69 ± 0.02cm3s-1 for C6D6+and k(T) = 2.0 ± 0.6 × 10-6 (T/300)−0.83 ± 0.02cm3s-1 for C6D7+.These expressions correlates well with earlier flowing afterglow studies on the sameprocess.

The mid-infrared emission of Polycyclic Aromatic Hydrocarbons is found in many phases ofthe interstellar medium. Towards cold dense clouds,however,the emission is heavilyquenched. In these regions molecules are found to efficiently freeze-out on interstellargrains forming thin layers of ices. PAHs are highly non-volatile molecules and are alsoexpected to freeze-out. PAHs trapped in interstellar ices are likely to participate in theoverall chemistry,leading to the formation of cations and complex molecules in thesolid-state. The work presented here aims to experimentally study the chemical reactionsthat PAHs undergo upon vacuum ultraviolet irradiation when trapped in interstellarH2O ice.

I give an overview of the recent scientific results based on observations of PAH emissionfrom circumstellar disks around young stars. The stellar radiation field plays a key rolein the excitation and destruction of the PAH molecules in the disk. The detection rate ofPAH emission in disks is optimal for stars of spectral type A. Around stars of similartemperature,the disk structure determines the PAH emission strength: disks with a flaredgeometry produce stronger PAH emission than flattened disks. The spectral properties ofthe emission features,indicative of the chemistry of the emitting hydrocarbons,isclosely linked to the central star radiation field. The main PAH features shift to redderwavelengths with decreasing stellar effective temperature. This trend has been interpretedas an indication for a higher aliphatic/aromatic ratio of the hydrocarbon mixture aroundcool stars with respect to hot stars. An alternative explanation may be a more significantcontribution to the infrared emission of very small grains around cooler stars.

Depending on whom you ask, PAHs are either the smallest dust particles or the largestgas-phase molecules in space. Whether referred to as gas or dust, these PAHs can containup to20% of the total cosmic carbon abundance and as such also play an important role inthe carbon chemistry of protoplanetary disks. The interpretation of PAH bands is often acomplex procedure involving not only gas physics to determine their ionization stage andtemperature, but also radiative transfer effects that can bury these bands in a strongthermal continuum from a population of larger dust particles.

PAHs are most readily seen in the spectral energy distributions (SEDs) of disks aroundHerbig AeBe stars where they are photoprocessed by the stellar radiation field. Resolvedimages taken in the PAH bands confirm their origin in the flaring surfaces ofcircumstellar disks: if the SED is consistent with a flat disk structure (lessilluminated), there is little or no evidence of PAH emission. The very low detection ratesin the disks around T Tauri stars often require an overall lower abundance of PAHs inthese disk surface as compared to that in molecular clouds.

In this review, I will adress three aspects of PAHs in protoplanetary disks: (1) Do PAHsform in protoplanetary disks or do they originate from the precursor molecular cloud? (2)Is the presence of PAH features in SEDs a consequence of the disk structure or do PAHs infact shape the disk structure? (3) How can we use PAHs as tracers of processes inprotoplanetary disks?

We present a Monte Carlo (MC) radiative transfer code for complex three dimensional dust distributions and include transiently heated PAH. The correctness of the code is confirmed by comparison with benchmark results. The method makes use of the parallelization capabilities of modern vectorized computing units like graphic cards. The computational speed grows linearly with the number of graphical processing units (GPU). On a conventional desktop PC, our code is up to a factor 100 faster when compared to other MC algorithms. As an example, we compute the dust emission of proto-planetary disks. We simulate how a mid-IR instrument mounted at a future 42 m ELT will detect such disks. Two cases are distinguished: a homogeneous disk and a disk with an outward migrating planet, producing a gap and a spiral density wave. We find that the resulting mid-IR spectra of both disks are almost identical. However, they can be distinguished at those wavelengths by coronographic, dual-band imaging. Finally, the emission of PAHs exposed to different radiation fields is computed. We demonstrate that PAH emission depends not only on the strength but also strongly on the hardness of the radiation, a fact which has often been neglected in previous models. We find that hard photons (>20  eV) easily dissociate all PAHs in the disks of T Tauri stars. To explain the low, but not negligible detection rate (<10%) of PAHs in T Tau disks, we suggest that turbulent motions act as a possible path for PAH survival.

Carbonaceous grains represent a major component of cosmic dust. The review will summarizenew results in laboratory investigations of carbonaceous dust components. Thenanometer-sized carbon particles are supposed to represent a blend of differentlystructured carbon including graphitic, diamond-like, fullerene-like and chain-likecomponents on a subnanometer or nanometer scale. Recent models used to explain thestructure of gas-phase condensed carbon nano-particles are discussed. Possible formationpathways of carbonaceous grains from molecular components and clusters and the role ofpolycyclic aromatic hydrocarbons (PAHs) and fullerenes are disclosed.

In dense molecular clouds, the birthplace of stars and planets, interstellar atoms andmolecules freeze onto extremely cold dust and ice particles. These ices are processed byultraviolet light and cosmic rays forming hundreds of far more complex species, some ofastrobiological interest. Eventually, these rain down on primordial planets where theytake part in the young chemistry on these new worlds.

Although the IR spectroscopy and energetic processing of interstellar ice analogs havebeen studied for nearly 30 years, similar studies of PAH containing ices have only justbegun. This paper presents recent results from laboratory studies on the vacuumultraviolet (VUV) photochemistry of PAHs in water ice at low temperatures to assess theroles they play in the photochemical evolution of interstellar ices and their relevance toastrobiology. A number of “surprises” were found in these studies on PAH containingwater-rich ices, indicating that PAHs likely play very important, unexpected roles incosmic ice chemistry, physics and astrobiology.

Lunar minerals and impact glasses, convert the polyatomic beam of solar wind (SW) ionsinto a flux of small molecules (e.g., H2, N2,H2O, CO, CO2, CH4, C2H4,C2H6, HCN, metal carbides and deuterides, etc.). They thus behaveas “Solid State Molecular Reactors”. Moreover,  ~100–200   μmsize micrometeoroids (μMs) have also been exposed to the SW in thezodiacal cloud, before being captured by the Earth and recovered as Antarcticmicrometeorites. They are mostly composed of a PAH-rich hydrous-carbonaceous material,which amplifies their power as molecular reactors. In particular, during the first ~200 Myr of the post-lunar period, about 75% of the μMs havebeen melted and/or volatilized upon atmospheric entry. The release of their volatilespecies triggered a cosmic volcanism around the mesopause that ruled the formation of theearly Earth’s atmosphere and climate. Furthermore, a fraction of the μMsthat survive unmelted upon atmospheric entry did settle on the proto-oceans floors. Uponfurther burial in sediments their constituent PAH-rich kerogen was cracked into abioticoil, which generated giant oil slicks that fed prebiotic chemistry. Many stars, of allages and types, are embedded into a secondary debris-disk loaded with ion implantedμMs. Some of them are expelled to the interstellar medium (ISM) wherethey behave first as “dormant-invisible” molecular reactors, until they became reactivatedby various processes to synthesize interstellar molecules. This short paper only focus onsome highlights of this research dealing with the synthesis of important interstellarmolecules, including the most abundant ones (H2 and CO) and H2O, HCNand PAHs, all involved in prebiotic chemistry.

Aromatic carbon, in some form, has been an essential ingredient by and large in allmodels of the extinction curve, since the original proposal to attribute the bump at 217.5nm to “astronomical graphite”. This aromatic carbon is most naturally identified, in up todate models, with a population of Polycyclic Aromatic Hydrocarbons (PAHs), free and/orclustered. In all models, this PAH population accounts for the far–UV nonlinear rise inthe extinction curve, contributes to the bump and possibly part of the large set ofunidentified, discrete absorption features in the visible (the Diffuse InterstellarBands). We review the current state of our understanding of the contribution of PAHs tointerstellar extinction, and what constraints can be imposed on the PAH population byfitting extinction models to observations.

The diffuse interstellar bands (DIBs) have been known of since 1922, but their carriermolecules remain unidentified to this day. We present a brief history of DIB observations,followed by a list of constraints any suggested origin must face, and finally a preview ofcurrent research for ultraviolet DIBs using the Cosmic Origins Spectrographon the Hubble Space Telescope. We conclude that PAHs areconsistent with all the listed constraints, but that PAHs may not be the only molecularspecies responsible for the DIBs.

In this contribution I review the PAH-DIB hypothesis. Firstly, I list several propertiesof PAHs and their behaviour and expected signatures in space. Next, I give an overview ofthe current observational insights on the DIB carriers. I conclude with a briefdescription of recent results on the search for the DIB carriers and the prospects we havefor identification.

Polycyclic aromatic hydrocarbons are a class of molecules of broad interest that has longbeen explored by various spectroscopic techniques. The electronic spectroscopy of thesespecies is of particular interest since it provides a framework for the understanding ofthe electronic structure of large polyatomic molecules. Such studies also allow thesystematic investigation of electronic relaxation mechanisms in large molecules. In thisreview, we focus on the gas-phase experimental work on such systems and present the latestprogress. We also underline the challenges that remain to be tackled. A focus on theunderstanding of the electronic relaxation pathways at work in gas-phase PAHs will also bepresented, as well as their possible manifestation in space.

The spectroscopic properties of protonated and deprotonated PAHs are investigated throughDensity Functional Theory (DFT) calculations, with reference to their potentialastrophysical significance. Attention is focussed on electronic and rotationalspectra.

Infrared absorption and emission features observed spectroscopically in our Galaxy allowto probe the composition of solid dust grains, their evolution and thus follow the cyclingof matter in the Galaxy. Many observables do reveal the presence of large amounts ofcarbonaceous particles in space, other than the PAH-like emission lines. The carbonaceousmaterials observed include amorphous carbons, diamondoids showing in emission for a fewspecific sources, and the recently detected fullerenes. An important hydrogenatedamorphous carbon component (HAC or a-C:H), traced by the 2940 cm-1 structuredabsorption feature is observed against Galactic background sources. Since the discovery ofthis feature in the early eighties (Allen 1981), theobservation of a-C:H has been extended to the mid-infrared by space observatories, givinginsight into additional associated features. They are also observed in external galaxies,showing the ubiquitous nature of these components. We will focus on astronomicalobservations of organic matter other than PAHs, amorphous carbons and associatedlaboratory dust analogues relevant to astrophysical applications.