Astronomical Polycyclic Aromatic Hydrocarbon Research Articles

Aliphatics and Aromatics in the Universe: The Pre-JWST Era

The so-called “unidentified infrared emission” (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm ubiquitously seen in a wide variety of astrophysical regions are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. Astronomical PAHs often have an aliphatic component (e.g., aliphatic sidegroups like methyl –CH3 may be attached as functional groups to PAHs) as revealed by the detection in many UIE sources of the aliphatic C–H stretching feature at 3.4 μm. With its unprecedented sensitivity and spatial resolution, and high spectral resolution, the James Webb Space Telescope (JWST) holds great promise for revolutionizing the studies of aliphatics and aromatics in the Universe. To facilitate analyzing JWST observations, we present a theoretical framework for determining the aliphatic fractions (η ali) of PAHs (the fractions of C atoms in aliphatic units) from the emission intensity ratios of the 3.4 μm aliphatic C–H feature to the 3.3 μm aromatic C–H feature. To demonstrate the effectiveness of this framework, we compile the 3.3 and 3.4 μm UIE data obtained in the pre-JWST era for an as-complete-as-possible sample, and then apply the framework to these pre-JWST data. We derive a median aliphatic fraction of 〈η ali〉 ≈ 5.4%, and find that the aliphatic fractions are the highest in protoplanetary nebulae illuminated by cool stars lacking ultraviolet radiation. Nevertheless, the “hardness” of stellar photons is not the only factor affecting the PAH aliphaticity, and other factors such as the starlight intensity may also play an important role.

Anharmonic IR absorption spectra of the prototypical interstellar PAHs phenanthrene, pyrene, and pentacene in their neutral and cation states

Combination band and overtone transitions of polycyclic aromatic hydrocarbon (PAH) cations in the 2000–2900 cm-1 (5-3.5 µm) region are implicated as carriers of the ‘quasi-continuum’ observed in the near-infrared (IR) emission spectra of many astronomical objects. In neutral PAHs, the strongest absorption features are concentrated in the 700–900 cm-1 (14–11 µm) range, which are associated with CH out-of-plane bending motions. Upon ionization, this shifts to the 1000–1600 cm-1 (10–6 µm) range, where bands are associated with C=C stretches and CH in-plane bends. Anharmonicity is required to accurately characterize the IR absorption spectrum of PAHs, indicated herein by the ability to directly assign the bands in high-resolution experimental absorption spectra of neutral and cationic phenanthrene, pyrene, and pentacene. Neutral PAHs are indicated as the source of the strong 3.3 and 11.2 µm astronomical PAH features, while the broad features in the 6–10 µm region and the ‘quasi-continuum’ from 3.5–5 µm stem from PAH cations. This study reinforces the need for including anharmonicity in the computation of IR absorption and emission spectra of larger and more complex PAHs. This is particularly pertinent to the interpretation of data returned by JWST.

Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C–H Stretching Modes and Astrophysical Implications

Abstract The so-called unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and ubiquitously seen in a wide variety of astrophysical regions are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. Astronomical PAHs may have an aliphatic component, as revealed by the detection in many UIE sources of the aliphatic C–H stretching feature at . The ratio of the observed intensity of the feature to that of the aromatic C–H feature allows one to estimate the aliphatic fraction of the UIE carriers. This requires knowledge of the intrinsic oscillator strengths of the aromatic C–H stretch ( ) and the aliphatic C–H stretch ( ). Lacking experimental data on and for the UIE candidate materials, one often has to rely on quantum-chemical computations. Although the second-order Møller–Plesset (MP2) perturbation theory with a large basis set is more accurate than the B3LYP density functional theory, MP2 is computationally very demanding and impractical for large molecules. Based on methylated PAHs, we show here that, by scaling the band strengths computed at an inexpensive level (e.g., B3LYP/6-31G*), we are able to obtain band strengths as accurate as those computed at far more expensive levels (e.g., MP2/6-311+G(3df,3pd)). We calculate the model spectra of methylated PAHs and their cations excited by starlight of different spectral shapes and intensities. We find that , the ratio of the model intensity of the feature to that of the feature, is insensitive to the spectral shape and intensity of the exciting starlight. We derive a straightforward relation for determining the aliphatic fraction of the UIE carriers (i.e., the ratio of the number of C atoms in aliphatic units to that in aromatic rings ) from the observed band ratios : for neutrals and for cations.

Polycyclic Aromatic Hydrocarbons in Protoplanetary Disks around Herbig Ae/Be and T Tauri Stars

Abstract A distinct set of broad emission features at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 μm, is often detected in protoplanetary disks (PPDs). These features are commonly attributed to polycyclic aromatic hydrocarbons (PAHs). We model these emission features in the infrared spectra of 69 PPDs around 14 T Tauri and 55 Herbig Ae/Be stars in terms of astronomical PAHs. For each PPD, we derive the size distribution and the charge state of the PAHs. We then examine the correlations of the PAH properties (i.e., sizes and ionization fractions) with the stellar properties (e.g., stellar effective temperature, luminosity, and mass). We find that the characteristic size of the PAHs tends to correlate with the stellar effective temperature ( ) and interpret this as the preferential photodissociation of small PAHs in systems with higher of which the stellar photons are more energetic. In addition, the PAH size shows a moderate correlation with the red-ward wavelength shift of the 7.7 μm PAH feature that is commonly observed in disks around cool stars. The ionization fraction of PAHs does not seem to correlate with any stellar parameters. This is because the charging of PAHs depends on not only the stellar properties (e.g., , luminosity) but also their spatial distribution in the disks. The marginally negative correlation between PAH size and stellar age suggests that continuous replenishment of PAHs via the outgassing of cometary bodies and/or the collisional grinding of planetesimals and asteroids is required to maintain the abundance of small PAHs against complete destruction by photodissociation.

BREAKDOWN PRODUCTS OF GASEOUS POLYCYCLIC AROMATIC HYDROCARBONS INVESTIGATED WITH INFRARED ION SPECTROSCOPY

ABSTRACT We report on a common fragment ion formed during the electron-ionization-induced fragmentation of three different three-ring polycyclic aromatic hydrocarbons (PAHs), fluorene (C13H10), 9,10-dihydrophenanthrene (C14H12), and 9,10-dihydroanthracene (C14H12). The infrared spectra of the mass-isolated product ions with m/z = 165 were obtained in a Fourier transform ion cyclotron resonance mass spectrometer whose cell was placed inside the optical cavity of an infrared free-electron laser, thus providing the high photon fluence required for efficient infrared multiple-photon dissociation. The infrared spectra of the m/z = 165 species generated from the three different precursors were found to be similar, suggesting the formation of a single isomer. Theoretical calculations using density functional theory (DFT) revealed the fragment's identity as the closed-shell fluorenyl cation. Decomposition pathways from each parent precursor to the fluorenyl ion are proposed on the basis of DFT calculations. The identification of a single fragmentation product from three different PAHs supports the notion of the existence of common decomposition pathways of PAHs in general and can aid in understanding the fragmentation chemistry of astronomical PAH species.

PROPERTIES OF POLYCYCLIC AROMATIC HYDROCARBONS IN THE NORTHWEST PHOTON DOMINATED REGION OF NGC 7023. I. PAH SIZE, CHARGE, COMPOSITION, AND STRUCTURE DISTRIBUTION

Polycyclic aromatic hydrocarbon (PAH) emission in the Spitzer Infrared Spectrograph spectral map of the northwest photon dominated region (PDR) in NGC 7023 was analyzed exclusively using PAH spectra from the NASA Ames PAH IR Spectroscopic Database (www.astrochem.org/pahdb). The 5-15 μm spectrum at each pixel is fitted using a non-negative-least-squares fitting approach. The fits are of good quality, allowing decomposition of the PAH emission into four subclasses: size, charge, composition, and hydrogen adjacency (structure). Maps tracing PAH subclass distributions across the region paint a coherent astrophysical picture. Once past some 20 seconds of arc from HD 200775, the emission is dominated by the more stable, large, symmetric, compact PAH cations with smaller, neutral PAHs taking over along the lines-of-sight toward the more distant molecular cloud. The boundary between the PDR and the denser cloud material shows up as a distinct discontinuity in the breakdown maps. Noteworthy is the requirement for PANH cations to fit the bulk of the 6.2 and 11.0 μm features and the indication of PAH photo-dehydrogenation and fragmentation close to HD 200775. Decomposition of the spectral maps into principal subclass template spectra provides additional insight into the behavior of each subclass. However, the general applicability of this computationally more efficient approach is presently undetermined. This is the first time the spectra of individual PAHs are exclusively used to fit the 5-15 μm region and analyze the spatial behavior of the aromatic infrared bands, providing fundamental, new information about astronomical PAH subpopulations including their dependence on, and response to, changes in local conditions.

THE INFRARED SPECTROSCOPY OF POLYCYCLIC AROMATIC HYDROCARBONS WITH FIVE- AND SEVEN-MEMBERED FUSED RING DEFECTS

Polycyclic aromatic hydrocarbon (PAH) growth and destruction are thought to proceed via the occasional incorporation of five- and seven-membered fused ring defects in the hexagonal carbon skeleton. Using density functional theory, this paper investigates the effect such five- and seven-membered fused ring defects have on the infrared spectra of ovalene, circumovalene, and circumcircumovalene. The defects make only small changes to the overall infrared (IR) spectra, both in the mid-IR and in the far-IR, of these species. In addition to small shifts in the positions of the bands between the PAHs with and without defects, the most common effect of the defects is to increase the number of bands. Except for an anion with the Stone-Wales defect, all of the species studied have the C-C stretching band at 6.3 μm or at longer wavelengths, the position in Classes B and C astronomical PAH spectra. In the case of the Stone-Wales anion, the band falls at 6.20 μm, suggesting that further study of defects is probably worthwhile, as some PAHs with defects might be important in those sources (Class A) that show a C-C stretching band that falls near 6.2 μm.

The 15–20 μm PAH emission features: probes of individual PAHs?

Context. Spectral features between about 15−20 μm are commonly associated with polycyclic aromatic hydrocarbons (PAHs). With the NASA Spitzer Space Telescope these features are reported routinely, and as such, warrant a deeper molecular explanation. Aims. We aim to determine the characteristics of the group of carriers of the plateau and the distinct sub-features at 15.8, 16.4, 17.4, 17.8 and 18.9 μm and to draw astronomical implications from these spectra. Methods. We analyse and interpret the spectra of 15 different sources using the NASA Ames PAH IR spectroscopic database. Results. The bands within the 15−20 μm region show large variations. Except for the 16.4 μm band, there is also no connection, both in band strength and feature classification, with the mid-IR PAH bands. Of the PAH spectra considered, only those from species containing pendent rings show one “common” characteristic: a band near the astronomical 16.4 μm position. However, coupling with the carbon skeleton’s core influences its precise position in the spectrum. Compact PAHs in the size range 50−130 carbon atoms, consistently show a strong band near the astronomical 17.4 μm band position. Conclusions. The 15−20 μm region is the transition zone from PAH nearest neighbour modes to full-skeleton modes. We conclude that a few individual PAHs dominate the astronomical PAH family when clear features are prominent. In the few cases of a broad plateau, the PAH family would be far richer. Although PAHs containing pendent rings showed promise explaining the astronomical 16.4 μm band, coupling with the skeleton core and the inherent strong quartet mode expected around 13.5 μm, make it a less viable candidate. The number of large PAHs in the database becomes a limitation in studying the emission between 15−20 μm and longward. Computation of larger PAH spectra should therefore be stimulated, especially for understanding the forthcoming far-IR data expected from Herschel, SOFIA and ALMA.

THE 5.25 AND 5.7 μm ASTRONOMICAL POLYCYCLIC AROMATIC HYDROCARBON EMISSION FEATURES

Astronomical mid-IR spectra show two minor polycyclic aromatic hydrocarbon (PAH) features at 5.25 and 5.7 μm (1905 and 1754 cm–1) that hitherto have been little studied, but contain information about the astronomical PAH population that complements that of the major emission bands. Here, we report a study involving both laboratory and theoretical analysis of the fundamentals of PAH spectroscopy that produce features in this region and use these to analyze the astronomical spectra. The Infrared Space Observatory Short Wavelength Spectrograph spectra of 15 objects showing these PAH features were considered for this study, however only four (HD 44179; NGC 7027; Orion Bar, two positions) have sufficient signal-to-noise between 5 and 6 μm to allow for an in-depth analysis. All four astronomical spectra show similar peak positions and profiles. The 5.25 μm feature is peaked and asymmetric, with an FWHM of about 0.12 ± 0.01 μm (~40 ± 6.5 cm–1), while the 5.7 μm feature is broader and flatter, with an FWHM of about 0.17 ± 0.02 μm (50 ± 5.6 cm–1). Detailed analysis of the laboratory spectra and quantum-chemical calculations show that the astronomical 5.25 and 5.7 μm bands are a blend of combination, difference and overtone bands primarily involving CH stretching and CH in-plane and CH out-of-plane bending fundamental vibrations. The experimental and computational spectra show that, of all the hydrogen adjacency classes that are possible on PAHs, solo and duo hydrogens consistently produce prominent bands at the observed positions, whereas quartet hydrogens do not. In all, this study supports the picture that astronomical PAHs are large with compact, regular structures. From the coupling with primarily strong CH out-of-plane bending modes, one might surmise that the 5.25 and 5.7 μm bands track the neutral PAH population. However, theory suggests that the role of charge in these astronomical bands might also be important.

Do the Infrared Emission Features Need Ultraviolet Excitation? The Polycyclic Aromatic Hydrocarbon Model in UV‐poor Reflection Nebulae

One of the major challenges to identification of the 3.3, 6.2, 7.7, 8.6, and 11.3 μm interstellar infrared (IR) emission bands with polycyclic aromatic hydrocarbon (PAH) molecules has been the recent detection of these bands in regions with little ultraviolet (UV) illumination, since small, neutral PAH molecules have little or no absorption at visible wavelengths and therefore require UV photons for excitation. We show here that our astronomical PAH model, incorporating the experimental result that the visual absorption edge shifts to longer wavelength upon ionization and/or as the PAH size increases, can closely reproduce the observed IR emission bands of vdB 133, a UV-poor reflection nebula. We also show that single-photon heating of astronomical PAHs in reflection nebulae near stars as cool as Teff = 3000 K can result in observable emission at 6.2, 7.7, 8.6, and 11.3 μm. Illustrative mid-IR emission spectra are calculated for reflection nebulae illuminated by cool stars with Teff = 3500, 4500, and 5000 K. These will allow comparison with future Space Infrared Telescope Facility observations of vdB 135 (Teff = 3600 K), vdB 47 (Teff = 4500 K), and vdB 101 (Teff = 5000 K). The dependence on the effective temperature of the exciting star of the observed 12 μm IRAS emission (relative to the total far-IR emission) is consistent with the PAH model for 3000 K ≤ Teff ≤ 30,000 K.