Fullerenes have recently been detected in various circumstellar and interstellar environments, raising the question of their formation pathway. It has been proposed that they can form at the low ...densities found in the interstellar medium 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 modelling, the possibility that the PAH C66H20 (i.e. circumovalene) can lead to the formation of the C60 fullerene upon irradiation by ultraviolet photons. The chemical pathway involves full dehydrogenation of C66H20, folding into a floppy closed cage and shrinking of the cage by loss of C2 units until it reaches the symmetric C60 molecule. At 10′′ from the illuminating star and with realistic molecular parameters, the model predicts that 100% of C66H20 is converted into C60 in ~105 yr, 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 C66H20 are unlikely to contribute significantly to the formation of C60, while PAHs containing between 60 and 66 C atoms should contribute to the formation of C60 with shorter timescales, and PAHs containing fewer than 60 C atoms will be destroyed. Assuming a classical size distribution for the PAH precursors, our model predicts that absolute abundances of C60 are up to several 10-4 of the elemental carbon, that is, less than a percent of the typical interstellar PAH abundance, which is consistent with observational studies. According to our model, once formed, C60 can survive much longer (> 107 yr for radiation fields below G0 = 104) than other fullerenes because of the remarkable stability of the C60 molecule at high internal energies. Hence, a natural consequence is that C60 is more abundant than other fullerenes in highly irradiated environments.
Context. Carbon chemistry plays a pivotal role in the interstellar medium (ISM) but even the synthesis of the simplest hydrocarbons and how they relate to polycyclic aromatic hydrocarbons (PAHs) and ...grains is not well understood. Aims. We study the spatial distribution and chemistry of small hydrocarbons in the Orion Bar photodissociation region (PDR), a prototypical environment in which to investigate molecular gas irradiated by strong UV fields. Methods. We used the IRAM 30 m telescope to carry out a millimetre line survey towards the Orion Bar edge, complemented with ~2′ × 2′ maps of the C2H and c-C3H2 emission. We analyse the excitation of the detected hydrocarbons and constrain the physical conditions of the emitting regions with non-LTE radiative transfer models. We compare the inferred column densities with updated gas-phase photochemical models including 13CCH and C13CH isotopomer fractionation. Results. Approximately 40% of the lines in the survey arise from hydrocarbons (C2H, C4H, c-C3H2, c-C3H, C13CH, 13CCH, l-C3H, and l-H2C3 in decreasing order of abundance). We detect new lines from l-C3H+ and improve its rotational spectroscopic constants. Anions or deuterated hydrocarbons are not detected, but we provide accurate upper limit abundances: C2D/C2H < 0.2%, C2H−/C2H < 0.007%, and C4H−/C4H < 0.05%. Conclusions. Our models can reasonably match the observed column densities of most hydrocarbons (within factors of <3). Since the observed spatial distribution of the C2H and c-C3H2 emission is similar but does not follow the PAH emission, we conclude that, in high UV-flux PDRs, photodestruction of PAHs is not a necessary requirement to explain the observed abundances of the smallest hydrocarbons. Instead, gas-phase endothermic reactions (or with barriers) between C+, radicals, and H2 enhance the formation of simple hydrocarbons. Observations and models suggest that the C2H/c-C3H2 ratio (~32 at the PDR edge) decreases with the UV field attenuation. The observed low cyclic-to-linear C3H column density ratio (≤3) is consistent with a high electron abundance (xe) PDR environment. In fact, the poorly constrained xe gradient influences much of the hydrocarbon chemistry in the more UV-shielded gas. The inferred hot rotational temperatures for C4H and l-C3H+ also suggest that radiative IR pumping affects their excitation. We propose that reactions of C2H isotopologues with 13C+ and H atoms can explain the observed C13CH/13CCH = 1.4 ± 0.1 fractionation level.
Various studies have emphasised variations in the charge state and composition of the interstellar polycyclic aromatic hydrocarbon (PAH) population in photodissociation regions (PDRs). These changes ...are expected to affect the energetics and chemistry in these regions, thereby calling for a quantitative description. 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, for which many observational constraints are available. We also discuss the case of the diffuse interstellar medium (ISM). We have developed a new model of PAH chemical evolution based on the most recent available molecular data. This model allows us to rationalise the observational constraints without any fitting parameter. 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.
Context. In photo-dissociation regions (PDRs), polycyclic aromatic hydrocarbons (PAHs) may be produced by evaporation of very small grains (VSGs) by the impinging UV radiation field from a nearby ...star. Aims. We quantitatively investigate the transition zone between evaporating VSGs (eVSGs) and PAHs in several PDRs. Methods. We studied the relative contribution of PAHs and eVSGs to the mid-IR emission in a wide range of excitation conditions. We fitted the observed mid-IR emission of PDRs by using a set of template band emission spectra of PAHs, eVSGs, and gas lines. The fitting tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the community as an IDL routine. From the results of the fit, we derived the fraction of carbon feVSG locked in eVSGs and compared it to the intensity of the local UV radiation field. Results. We show a clear decrease of feVSG with increasing intensity of the local UV radiation field, which supports the scenario of photo-destruction of eVSGs. Conversely, this dependence can be used to quantify the intensity of the UV radiation field for different PDRs, including unresolved ones. Conclusions. PAHTAT can be used to trace the intensity of the local UV radiation field in regions where eVSGs evaporate, which correspond to relatively dense (nH = 100,105 cm-3) and UV irradiated PDRs (G0 = 100,5 × 104 ) where H2 emits in rotational lines.
Context. In bright photodissociation regions (PDR) associated with massive star formation, the presence of dense “clumps” that are immersed in a less dense interclump medium is often proposed to ...explain the difficulty of models to account for the observed gas emission in high-excitation lines. Aims. We aim to present a comprehensive view of the modelling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces. Methods. We observed the 12CO and 13CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel. We also considered line emission from key species in the gas cooling of PDRs (C+, O, and H2) and other tracers of PDR edges such as OH and CH+. All the intensities are collected from Herschel observations, the literature and the Spitzer archive and were analysed using the Meudon PDR code. Results. A grid of models was run to explore the parameter space of only two parameters: thermal gas pressure and a global scaling factor that corrects for approximations in the assumed geometry. We conclude that the emission in the high-J CO lines, which were observed up to Jup = 23 in the Orion Bar (Jup = 19 in NGC 7023), can only originate from small structures with typical thicknesses of a few 10−3 pc and at high thermal pressures (Pth ~ 108 K cm−3). Conclusions. Compiling data from the literature, we find that the gas thermal pressure increases with the intensity of the UV radiation field given by G0, following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help to rationalise the analysis of high-J CO emission in massive star formation and provides an observational constraint for models which study stellar feedback on molecular clouds.
Emission of fullerenes in their infrared vibrational bands has been detected in space near hot stars. The proposed attribution of the diffuse interstellar bands at 9577 and 9632 Å to electronic ...transitions of the buckminsterfullerene cation (i.e. C60+) was recently supported by new laboratory data, confirming the presence of this species in the diffuse interstellar medium (ISM). In this Letter, we present the detection, also in the diffuse ISM, of the 17.4 and 18.9 μm emission bands commonly attributed to vibrational bands of neutral C60 . According to classical models that compute the charge state of large molecules in space, C60 is expected to be mostly neutral in the diffuse ISM. This is in agreement with the abundances of diffuse C60 we derive here from observations.
Aims. We observationally investigate the relation between the photoelectric heating efficiency in photodissociation regions (PDRs) and the charge of polycyclic aromatic hydrocarbons (PAHs), which are ...considered to play a key role in photoelectric heating. Methods. Using PACS onboard Herschel, we observed six PDRs spanning a wide range of far-ultraviolet radiation fields (G0 = 100−105). To measure the photoelectric heating efficiency, we obtained the intensities of the main cooling lines in these PDRs, i.e., the O i 63 μm, 145 μm, and C ii 158 μm, as well as the far-infrared (FIR) continuum intensity. We used Spitzer/IRS spectroscopic mapping observations to investigate the mid-infrared (MIR; 5.5−14 μm) PAH features in the same regions. We decomposed the MIR PAH emission into that of neutral (PAH0) and positively ionized (PAH+) species to derive the fraction of the positively charged PAHs in each region, and compare it to the photoelectric heating efficiency. Results. The heating efficiency traced by (O i 63 μm + O i 145 μm + C ii 158 μm)/TIR, where TIR is the total infrared flux, ranges between 0.1% and 0.9% in different sources, and the fraction of PAH+ relative to (PAH0+ PAH+) spans from 0 (+11)% to 87 (±10)%. All positions with a high PAH+ fraction show a low heating efficiency, and all positions with a high heating efficiency have a low PAH+ fraction, supporting the scenario in which a positive grain charge results in a decreased heating efficiency. Theoretical estimates of the photoelectric heating efficiency show a stronger dependence on the charging parameter γ = G0T1/2/ne than the observed efficiency reported in this study, and the discrepancy is significant at low γ. The photoelectric heating efficiency on PAHs, traced by (O i 63 μm + O i 145 μm + C ii 158 μm)/(PAH-band emission + O i 63 μm + O i 145 μm + C ii 158 μm), shows a much better match between the observations and the theoretical estimates. Conclusions. The good agreement of the photoelectric heating efficiency on PAHs with a theoretical model indicates the dominant contribution of PAHs to the photoelectric heating. This study demonstrates the fundamental role that PAHs have in photoelectric heating. More studies of their charging behavior are crucial to understand the thermal balance of the interstellar medium.
Context. The James Webb Space Telescope (JWST) will deliver an unprecedented quantity of high-quality spectral data over the 0.6−28 μm range. It will combine sensitivity, spectral resolution, and ...spatial resolution. Specific tools are required to provide efficient scientific analysis of such large data sets. Aims. Our aim is to illustrate the potential of unsupervised learning methods to get insights into chemical variations in the populations that carry the aromatic infrared bands (AIBs), more specifically polycyclic aromatic hydrocarbon (PAH) species and carbonaceous very small grains (VSGs). Methods. We present a method based on linear fitting and blind signal separation (BSS) for extracting representative spectra for a spectral data set. The method is fast and robust, which ensures its applicability to JWST spectral cubes. We tested this method on a sample of ISO-SWS data, which resemble most closely the JWST spectra in terms of spectral resolution and coverage. Results. Four representative spectra were extracted. Their main characteristics appear consistent with previous studies with populations dominated by cationic PAHs, neutral PAHs, evaporating VSGs, and large ionized PAHs, known as the PAHx population. In addition, the 3 μm range, which is considered here for the first time in a BSS method, reveals the presence of aliphatics connected to neutral PAHs. Each representative spectrum is found to carry second-order spectral signatures (e.g., small bands), which are connected with the underlying chemical diversity of populations. However, the precise attribution of theses signatures remains limited by the combined small size and heterogeneity of the sample of astronomical spectra available in this study. Conclusions. The upcoming JWST data will allow us to overcome this limitation. The large data sets of hyperspectral images provided by JWST analysed with the proposed method, which is fast and robust, will open promising perspectives for our understanding of the chemical evolution of the AIB carriers.
Context. A chemical scenario was proposed for photon-dominated regions (PDRs) according to which UV photons from nearby stars lead to the evaporation of very small grains (VSGs) and the production of ...gas-phase polycyclic aromatic hydrocarbons (PAHs). Aims. Our goal is to achieve better insight into the composition and evolution of evaporating very small grains (eVSGs) and PAHs through analyzing the infrared (IR) aliphatic and aromatic emission bands. Methods. We combined spectro-imagery in the near- and mid-IR to study the spatial evolution of the emission bands in the prototypical PDR NGC 7023. We used near-IR spectra obtained with the IRC instrument onboard AKARI to trace the evolution of the 3.3 μm and 3.4 μm bands, which are associated with aromatic and aliphatic C−H bonds on PAHs. The spectral fitting involved an additional broad feature centered at 3.45 μm that is often referred to as the plateau. Mid-IR observations obtained with the IRS instrument onboard the Spitzer Space Telescope were used to distinguish the signatures of eVSGs and neutral and cationic PAHs. We correlated the spatial evolution of all these bands with the intensity of the UV field given in units of the Habing field G0 to explore how their carriers are processed. Results. The intensity of the 3.45 μm plateau shows an excellent correlation with that of the 3.3 μm aromatic band (correlation coefficient R = 0.95) and a relatively poor correlation with the aliphatic 3.4 μm band (R = 0.77). This indicates that the 3.45 μm feature is dominated by the emission from aromatic bonds. We show that the ratio of the 3.4 μm and 3.3 μm band intensity (I3.4/I3.3) decreases by a factor of 4 at the PDR interface from the more UV-shielded layers (G0 ~ 150,I3.4/I3.3 = 0.13) to the more exposed layers (G0> 1 × 104,I3.4/I3.3 = 0.03). The intensity of the 3.3 μm band relative to the total neutral PAH intensity shows an overall increase with G0, associated with an increase of both the hardness of the UV field and the H abundance. In contrast, the intensity of the 3.4 μm band relative to the total neutral PAH intensity decreases with G0, showing that their carriers are actively destroyed by UV irradiation and are not efficiently regenerated. The transition region between the aliphatic and aromatic material is found to correspond spatially with the transition zone between neutral PAHs and eVSGs. Conclusions. We conclude that the photo-processing of eVSGs leads to the production of PAHs with attached aliphatic sidegroups that are revealed by the 3.4 μm emission band. Our analysis provides evidence for the presence of very small grains of mixed aromatic and aliphatic composition in PDRs.
The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Ultrashort extreme ultraviolet pulses offer a ...means to reveal molecular dynamics at ultrashort timescales. Here, we report the use of time-resolved electron-momentum imaging combined with extreme ultraviolet attosecond pulses to study highly excited organic molecules. We measure relaxation timescales that increase with the state energy. High-level quantum calculations show these dynamics are intrinsic to the time-dependent many-body molecular wavefunction, in which multi-electronic and non-Born-Oppenheimer effects are fully entangled. Hints of coherent vibronic dynamics, which persist despite the molecular complexity and high-energy excitation, are also observed. These results offer opportunities to understand the molecular dynamics of highly excited species involved in radiation damage and astrochemistry, and the role of quantum mechanical effects in these contexts.
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