Infrared Emission Features Research Articles

On the Origin of Infrared Bands Attributed to Tryptophan in Spitzer Observations of IC 348

Infrared emission features toward interstellar gas of the IC 348 star cluster in Perseus have been recently proposed to originate from the amino acid tryptophan. The assignment was based on laboratory infrared spectra of tryptophan pressed into pellets, a method which is known to cause large frequency shifts compared to the gas phase. We assess the validity of the assignment based on the original Spitzer data as well as new data from JWST. In addition, we report new spectra of tryptophan condensed in para-hydrogen matrices to compare with the observed spectra. The JWST MIRI data do not show evidence for tryptophan, despite deeper integration toward IC 348. In addition, we show that several of the lines attributed to tryptophan are likely due to instrumental artifacts. This, combined with the new laboratory data, allows us to conclude that there is no compelling evidence for the tryptophan assignment.

IR spectra of cationic 1,5,9-triazacoronene and two of its cationic derivatives.

Infrared emission features are observed towards diverse astronomical objects in the interstellar medium (ISM). Generally, the consensus is that these IR features originate from polycyclic aromatic hydrocarbons (PAHs) and are hence named aromatic infrared bands (AIBs). More recently, it has been suggested that nitrogen substituted PAHs (PANHs) contribute to the AIBs as well and it has even been shown that nitrogen inclusion in PAHs can improve the match with the AIBs, specifically around the 6.2 μm feature. In order to determine which specific molecules or functional groups are at the origin of the AIBs, IR spectra of various PA(N)Hs are experimentally and computationally studied. In this work we expand on the spectroscopic investigation of PANHs by presenting the gas-phase mid-IR spectra of 1,5,9-triazacoronene˙+ (TAC˙+, m/z 303), a threefold nitrogenated congener of coronene˙+, its protonated derivative TACH+ (m/z 304) and the product that forms when water adds to dehydrogenated TAC˙+[TAC-H + H2O]+ (m/z 320). We analyze the mid-IR spectra by comparing them with vibrational modes calculated at the B3LYP/6-311++G(d,p) level a theory and we present a possible geometry for [TAC-H + H2O]+. The TAC˙+ mid-IR spectrum is compared to that of coronene˙+ and is demonstrated to be remarkably similar. We put TAC˙+ and TACH+ into astronomical context by comparing their recorded mid-IR spectra to observed ISM spectra of four spectral classes. From this we conclude that TAC˙+ and TACH+ could both contribute to the AIBs, with TACH+ being a more likely contributor than TAC˙+.

Infrared Emission of Specific Polycyclic Aromatic Hydrocarbon Molecules: Cyanonaphthalenes

The unidentified infrared emission features at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 μm are ubiquitously seen in a wide variety of astrophysical regions and commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules. However, the unambiguous identification of any individual, specific PAH molecules has proven elusive until very recently, when two isomers of cyanonapthalene, which consists of two fused benzene rings and substitutes a nitrile (–CN) group for a hydrogen atom, were discovered in the Taurus Molecular Cloud, based on their rotational transitions at radio frequencies. To facilitate the James Webb Space Telescope (JWST) to search for cyanonapthalenes in astrophysical regions, we model the vibrational excitation of cyanonapthalenes and calculate their infrared emission spectra in a number of representative astrophysical regions. The model emission spectra and intensities will allow JWST to quantitatively determine or place an upper limit on the abundances of cyanonapthalenes.

Machine learning identified molecular fragments responsible for infrared emission features of polycyclic aromatic hydrocarbons

ABSTRACT Machine learning feature importance calculations are used to determine the molecular substructures that are responsible for mid- and far-infrared (IR) emission features of neutral polycyclic aromatic hydrocarbons (PAHs). Using the extended-connectivity fingerprint as a descriptor of chemical structure, a random forest model is trained on the spectra of 14 124 PAHs to evaluate the importance of 10 632 molecular fragments for each band within the range of 2.761 to $1172.745\, \mu$m. The accuracy of the results is confirmed by comparing them with previously studied unidentified infrared emission (UIE) bands. The results are summarized in two tables available as Supplementary Data, which can be used as a reference for assessing possible UIE carriers. We demonstrate that the tables can be used to explore the relation between the PAH structure and the spectra by discussing about the IR features of nitrogen-containing PAHs and superhydrogenated PAHs.

A Morphological Study of Two Young Multipolar Planetary Nebulae

We carry out an optical morphological and infrared spectral study for two young planetary nebulae (PNs) Hen 2-158 and Pe 1-1 to understand their complex shapes and dust properties. Hubble Space Telescope optical images reveal that these nebulae have several bipolar-lobed structures and a faint arc with a clear boundary is located at the northwestern side of Pe 1-1. The presence of this arc-shaped structure suggests that the object interacts with its nearby interstellar medium. Spitzer IRS spectroscopic observations of these young nebulae clearly show prominent unidentified infrared emission features and a weak silicate band in Pe 1-1, indicating that Hen 2-158 is a carbon-rich nebula and Pe 1-1 has a mixed chemistry dust environment. Furthermore, we construct two three-dimensional models for these PNs to realize their intrinsic structures. The simulated models of the nebulae suggest that multipolar nebulae may be more numerous than we thought. Our analyses of spectral energy distributions for Hen 2-158 and Pe 1-1 show that they have low luminosities and low stellar effective temperatures, suggesting that these nebulae are young PNs. A possible correlation between typical multipolar young PNs and nested nebulae is also discussed.

The 3.3 μm Infrared Emission Feature Profile in HD 44179

HD 44179 is a postasymptotic giant branch star with a biconical nebula known as the Red Rectangle. It shows the 3.3, 3.4, 6.2, 7.7, 8.6, and 11.3 μm infrared emission features (commonly known as the polycyclic aromatic hydrocarbon (PAH) emission bands and the Aromatic Infrared Bands). Previous observations of HD 44179 showed that the width of the 3.3 μm emission feature was narrower within 2″ of HD 44179 and wider further away from the star. This was interpreted as an indication that the 3.3 μm emission feature arises from two types of PAHs with spatially varying relative abundances. With new observations of HD 44179 taken with the NASA Infrared Telescope Facility we find that the width of the 3.3 μm emission feature profile is not narrower on HD 44179 and conclude that there is no evidence for two spatially varying components.

Machine-learning Interpretation of the Correlation between Infrared Emission Features of Interstellar Polycyclic Aromatic Hydrocarbons

Abstract Supervised machine-learning models are trained with various molecular descriptors to predict infrared (IR) emission spectra of interstellar polycyclic aromatic hydrocarbons. We demonstrate that a feature importance analysis based on the random forest algorithm can be utilized to explore the physical correlation between emission features. Astronomical correlations between IR bands are analyzed as examples of demonstration by finding the common molecular fragments responsible for different bands, which improves the current understanding of the long-observed correlations. We propose a way to quantify the band correlation by measuring the similarity of the feature importance arrays of different bands, by which a correlation map is obtained for emissions in the out-of-plane bending region. Moreover, a comparison between the predictions using different combinations of descriptors underscores the strong prediction power of the extended-connectivity molecular fingerprint, and shows that the combinations of multiple descriptors of other types in general lead to improved predictivity.

Laser-Induced Electronic and Vibronic Dynamics in the Pyrene Molecule and Its Cation.

Among polycyclic aromatic hydrocarbons, pyrene is widely used as an optical probe thanks to its peculiar ultraviolet absorption and infrared emission features. Interestingly, this molecule is also an abundant component of the interstellar medium, where it is detected via its unique spectral fingerprints. In this work, we present a comprehensive first-principles study on the electronic and vibrational response of pyrene and its cation to ultrafast, coherent pulses in resonance with their optically active excitations in the ultraviolet region. The analysis of molecular symmetries, electronic structure, and linear optical spectra is used to interpret transient absorption spectra and kinetic energy spectral densities computed for the systems excited by ultrashort laser fields. By disentangling the effects of the electronic and vibrational dynamics via ad hoc simulations with stationary and moving ions, and, in specific cases, with the aid of auxiliary model systems, we rationalize that the nuclear motion is mainly harmonic in the neutral species, while strong anharmonic oscillations emerge in the cation, driven by electronic coherence. Our results provide additional insights into the ultrafast vibronic dynamics of pyrene and related compounds and set the stage for future investigations on more complex carbon-conjugated molecules.

Polycyclic aromatic hydrocarbons in Seyfert and star-forming galaxies

ABSTRACT Polycyclic Aromatic Hydrocarbons (PAHs) are carbon-based molecules resulting from the union of aromatic rings and related species, which are likely responsible for strong infrared emission features. In this work, using a sample of 50 Seyfert galaxies (DL < 100 Mpc) we compare the circumnuclear (inner kpc) PAH emission of AGN to that of a control sample of star-forming galaxies (22 luminous infrared galaxies and 30 H ii galaxies), and investigate the differences between central and extended PAH emission. Using Spitzer/InfraRed Spectrograph spectral data of Seyfert and star-forming galaxies and newly developed PAH diagnostic model grids, derived from theoretical spectra, we compare the predicted and observed PAH ratios. We find that star-forming galaxies and AGN-dominated systems are located in different regions of the PAH diagnostic diagrams. This suggests that not only are the size and charge of the PAH molecules different, but also the nature and hardness of the radiation field that excite them. We find tentative evidence that PAH ratios in AGN-dominated systems are consistent with emission from larger PAH molecules (Nc > 300–400) as well as neutral species. By subtracting the spectrum of the central source from the total, we compare the PAH emission in the central versus extended region of a small sample of AGN. In contrast to the findings for the central regions of AGN-dominated systems, the PAH ratios measured in the extended regions of both type 1 and type 2 Seyfert galaxies can be explained assuming similar PAH molecular size distribution and ionized fractions of molecules to those seen in central regions of star-forming galaxies (100 < Nc < 300).

The 3.3 μm Infrared Emission Feature: Observational and Laboratory Constraints on Its Carrier

Abstract We examine the self-consistency of laboratory and observational data for potential carriers of the 3.3 μm infrared emission feature (IEF), a member of the ubiquitous family of strong interstellar IEFs at 3.3, 3.4, 6.2, 7.7, 8.6, 11.2, and 12.7 μm. Previous studies have shown that most Galactic sources (reflection nebulae, H ii regions, and planetary nebulae) show 3.3 μm IEFs displaying similar central wavelengths, FWHM, and profiles. Our study is focused on the band profile designated as Class A, the most prevalent of four classes of observed band profiles. In contrast to the observations, laboratory spectra for gas phase polycyclic aromatic hydrocarbons (PAHs), the widely assumed carriers of the IEFs, display central wavelength shifts, widths, and profiles that vary with temperature and PAH size. We present an extrapolation of the laboratory band shifts and widths for smaller PAHs (≤32 carbon atoms) to the larger PAHs (>50 carbon atoms) that are thought to be the IEF carriers. The extrapolation leads to tight constraints on the sizes of the putative PAH carriers. Reconciling the observations with the implications of the laboratory spectra pose a significant challenge to the PAH and other IEF carrier hypotheses.

Effects of anisotropy on absorption cross-section spectra of medium-sized spheroidal corundum particles

Context. It has been widely accepted that corundum particles condense in the atmospheres of oxygen-rich asymptotic giant branch stars and effectively produce an infrared emission feature at 13 μm. Laboratory experiments have predicted that these particles have the shape of oblate spheroids. Aims. We investigate the influence of the material anisotropy of uniaxial corundum on absorption cross section spectra of medium sized spheroidal particles in the infrared spectral region. Methods. We compared absorption cross-section spectra of the anisotropic corundum particles gained by finite-difference time-domain simulations to spectra calculated by a weighted sum approximation of the according fictive isotropic materials, with one material having the dielectric function of the a–b-plane and the other having the dielectric function of the c-axis of corundum. We carried out investigations for different axes ratios of the spheroids, particles volumes, and different geometries of the dielectric axes to the particle axes as well as to the polarization and propagation direction of the incident light. Results. We observed several effects attributed to anisotropy that are non-additive, so that they cannot be reproduced with the combined spectra of the isotropic materials. Conclusions. Care should be taken when calculating the corundum infrared spectrum with simpler approaches. When particle sizes above 1 μm are to be considered, the T-matrix formalism delivers correct band shifts and bulk modes for many, but not all bands. This remains true in orientation-averaged spectra and for particles in the 0.1 μm size range.

The 21 μm and 30 μm emission features in carbon-rich objects

We review the observational properties and possible carriers of the 21 and 30 μm features seen in the spectra of evolved carbon-rich objects. The 30 μm feature is generally taken to be due to MgS, possibly in combination with other sulfides. It is commonly assumed that the sulfides coat grains of SiC or amorphous carbon, but it remains unclear if coated grains can produce the observed feature given the low relative abundance of sulfur and the optical constraints on the thickness of the coating and size of the grains. The lack of optical constants for MgS at wavelengths below 1 μm hampers attempts to model the dust shells of these objects. A small number of alternative carriers have been proposed, but these have not been investigated thoroughly. The situation for the 21 μm feature is more ambiguous. Many carriers have been proposed, but each has issues, and none have gained broad support. The feature only appears in carbon-rich objects which have evolved off of the asymptotic giant branch but have not yet become planetary nebulae. The spectral class of the central star is typically F or G. The transient nature of the 21 μm feature is an important clue to the nature of its carrier. It appears with a variety of other infrared emission features related to complex hydrocarbons, including aliphatic species and polycyclic aromatic hydrocarbons, but correlations with specific features have proven difficult to quantify.

Overcoming the out-of-plane bending issue in an aromatic hydrocarbon: the anharmonic vibrational frequencies of c-(CH)C3H2+

The challenges associated with the out-of-plane bending problem in multiply-bonded hydrocarbon molecules can be mitigated in quartic force field analyses by varying the step size in the out-of-plane coordinates. Carbon is a highly prevalent element in astronomical and terrestrial environments, but this major piece of its spectra has eluded theoretical examinations for decades. Earlier explanations for this problem focused on method and basis set issues, while this work seeks to corroborate the recent diagnosis as a numerical instability problem related to the generation of the potential energy surface. Explicit anharmonic frequencies for c-(CH)C3H2+ are computed using a quartic force field and the CCSD(T)-F12b method with cc-pVDZ-F12, cc-pVTZ-F12, and aug-cc-pVTZ basis sets. The first of these is shown to offer accuracy comparable to that of the latter two with a substantial reduction in computational time. Additionally, c-(CH)C3H2+ is shown to have two fundamental frequencies at the onset of the interstellar unidentified infrared bands, at 5.134 and 6.088 μm or 1947.9 and 1642.6 cm-1, respectively. This suggests that the results in the present study should assist in the attribution of parts of these aromatic bands, as well as provide data in support of the laboratory or astronomical detection of c-(CH)C3H2+.

Unidentified infrared emission features in mid-infrared spectrum of comet 21P/Giacobini-Zinner

Comet 21P/Giacobini-Zinner (hereafter, comet 21P/G-Z) is a Jupiter-family comet and a parent comet of the October Draconids meteor shower. If meteoroids originating from a Jupiter-family comet contain complex organic molecules, such as amino acids, they are essential pieces of the puzzle regarding the origin of life on Earth. We observed comet 21P/G-Z in the mid-infrared wavelength region using the Cooled Mid-infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope on UT 2005 July 5. Here, we report the unidentified infrared (UIR) emission features of comet 21P/G-Z, which are likely due to complex organic molecules (both aliphatic and aromatic hydrocarbons), and the thermal emission from amorphous/crystalline silicates and amorphous carbon grains in its mid-infrared low-resolution spectrum. The UIR features at ~8.2 μm, ~8.5 μm, and ~11.2 μm found in the spectrum of comet 21P/G-Z could be attributed to polycyclic aromatic hydrocarbons (or hydrogenated amorphous carbons) contaminated by N- or O-atoms, although part of the feature at ~11.2 μm comes from crystalline olivine. The other feature at ~9.2 μm might originate from aliphatic hydrocarbons. Comet 21P/G-Z is enriched in complex organic molecules. Considering that the derived mass fraction of crystalline silicates in comet 21P/G-Z is typical of comets, we propose that the comet originated from a circumplanetary disk of giant planets (similar to Jupiter and Saturn) where was warmer than the typical comet-forming region (5–30 au from the Sun) and was suitable for the formation of complex organic molecules. Comets from circumplanetary disks might be enriched in complex organic molecules, such as comet 21P/G-Z, and may have provided pre-biotic molecules to ancient Earth by direct impact or meteor showers.

Graphene oxide nanoparticles in the interstellar medium

ABSTRACT Dust particles play a major role in the formation, evolution and chemistry of interstellar clouds, stars, and planetary systems. Commonly identified forms include amorphous and crystalline carbon-rich particles and silicates. Also present in many astrophysical environments are polycyclic aromatic hydrocarbons (PAHs), detected through their infrared emission, and which are essentially small flakes of graphene. Astronomical observations over the past four decades have revealed a widespread unassigned ‘extended red emission’ (ERE) feature which is attributed to luminescence of dust grains. Numerous potential carriers for ERE have been proposed but none has gained general acceptance. In this Letter it is shown that there is a strong similarity between laboratory optical emission spectra of graphene oxide (GO) and ERE, leading to this proposal that emission from GO nanoparticles is the origin of ERE and that these are a significant component of interstellar dust. The proposal is supported by infrared emission features detected by the Infrared Space Observatory (ISO) and the Spitzer Space Telescope.

Formation and infrared identification of protonated fluoranthene isomers 3-, 9-, and 10-C16H11+ in solid para-H2.

Polycyclic aromatic hydrocarbons (PAH) and their derivatives are prospective carriers of unidentified infrared (UIR) emission features observed in interstellar media. Fluoranthene (C16H10) is a simple planar PAH with five- and six-membered rings; it can be considered as a fragment of C60, which, along with its cationic counterpart, has been identified in interstellar media. Protonated fluoranthene, C16H11+, was generated upon electron bombardment during deposition at 3.2 K of p-H2 containing fluoranthene in a small proportion. The intensities of infrared features of C16H11+ decreased after maintaining the matrix in darkness because of its neutralization with trapped electrons. According to the correlations in intensities upon neutralization and secondary photolysis, observed lines were classified into three groups which are assigned to isomers 3-C16H11+, 9-C16H11+, and 10-C16H11+. Experimental vibrational wavenumbers and relative IR intensities of the features agree with corresponding calculated values predicted for these three isomers of C16H11+ with the B3PW91/6-311++G(2d,2p) method. 3-C16H11+ and 9-C16H11+ are predicted to have the lowest energy (within 5 kJ mol-1), whereas 10- and 1-C16H11+ are lying above the global minimum 3-C16H11+ by ∼20 kJ mol-1. However, definitive identification of 1-C16H11+ could not be made as only the most intense line is tentatively assigned. Although the observed spectra of these isomers match unsatisfactorily with the UIR bands, they will facilitate the potential terrestrial and extraterrestrial identification of these species.

A specific interstellar aromatic molecule

Astrochemistry Aromatic molecules such as polycyclic aromatic hydrocarbons (PAHs) are known to exist in the interstellar medium owing to their characteristic infrared emission features. However, the infrared emission only indicates the general class of molecule, and identifying which specific molecular species are present is difficult. McGuire et al. used radio astronomy to detect rotational transitions of benzonitrile emitted from a well-known nearby cloud of interstellar gas (see the Perspective by Joblin and Cernicharo). This molecule may be a precursor to more complex PAHs. The identification of benzonitrile sheds light on the composition of aromatic material within the interstellar medium—material that will eventually be incorporated into new stars and planets. Science , this issue p. [202][1]; see also p. [156][2] [1]: /lookup/doi/10.1126/science.aao4890 [2]: /lookup/doi/10.1126/science.aar4541

On the Origin of the 3.3 μm Unidentified Infrared Emission Feature

Abstract The 3.3 μm unidentified infrared emission feature is commonly attributed to the C–H stretching band of aromatic molecules. Astronomical observations have shown that this feature is composed of two separate bands at 3.28 and 3.30 μm, and the origin of these two bands is unclear. In this paper, we perform vibrational analyses based on quantum mechanical calculations of 153 organic molecules, including both pure aromatic molecules and molecules with mixed aromatic/olefinic/aliphatic hydridizations. We find that many of the C–H stretching vibrational modes in polycyclic aromatic hydrocarbon (PAH) molecules are coupled. Even considering the uncoupled modes only, the correlation between the band intensity ratios and the structure of the PAH molecule is not observed, and the 3.28 and 3.30 μm features cannot be directly interpreted in the PAH model. Based on these results, the possible aromatic, olefinic, and aliphatic origins of the 3.3 μm feature are discussed. We suggest that the 3.28 μm feature is assigned to aromatic C–H stretch whereas the 3.30 μm feature is olefinic. From the ratio of these two features, the relative olefinic to aromatic content of the carrier can be determined.

Probing the infrared counterparts of diffuse far-ultraviolet sources in the Galaxy

Recent availability of high quality infrared (IR) data for diffuse regions in the Galaxy and external galaxies have added to our understanding of interstellar dust. A comparison of ultraviolet (UV) and IR observations may be used to estimate absorption, scattering and thermal emission from interstellar dust. In this paper, we report IR and UV observations for selective diffuse sources in the Galaxy. Using archival mid-infrared (MIR) and far-infrared (FIR) observations from Spitzer Space Telescope, we look for counterparts of diffuse far-ultraviolet (FUV) sources observed by the Voyager, Far Ultraviolet Spectroscopic Explorer (FUSE) and Galaxy Evolution Explorer (GALEX) telescopes in the Galaxy. IR emission features at 8µm are generally attributed to Polycyclic Aromatic Hydrocarbon (PAH) molecules, while emission at 24µm are attributed to Very Small Grains (VSGs). The data presented here is unique and our study tries to establish a relation between various dust populations. By studying the FUV-IR correlations separately at low and high latitude locations, we have identified the grain component responsible for the diffuse FUV emission.

Laboratory spectroscopy and astronomical significance of the fully-benzenoid PAH triphenylene and its cation

Triphenylene (C18H12) is a highly symmetric polycyclic aromatic hydrocarbon (PAH) molecule with a ‘fully-benzenoid’ electronic structure. This confers a high chemical stability compared with PAHs of similar size. Although numerous infrared and UV-vis experimental spectroscopic and theoretical studies of a wide range PAHs in an astrophysical context have been conducted, triphenylene and its radical cation have received almost no attention. There exists a huge body of spectroscopic evidence for neutral and ionised PAHs in astrophysical sources, obtained principally through detection of infrared emission features that are characteristic of PAHs as a chemical class. However, it has so far not proved possible to identify spectroscopically a single isolated PAH in space, although PAHs including triphenylene have been detected mass spectrometrically in meteorites. In this work we focus on recording laboratory electronic spectra of neutral and ionised triphenylene between 220 and 780 nm, trapped in H2O ice and solid argon at 12 K. The studies are motivated by the potential for spectroscopic astronomical detection of electronic absorption spectra of PAHs in ice mantles on interstellar grains as discussed by Linnartz (2014), and were performed also in a cold Ar matrix to provide guidance as to whether triphenylene (particularly in its singly positively ionised form) could be a viable candidate for any of the unidentified diffuse interstellar absorption bands. Based on the argon-matrix experimental results, comparison is made with previously unpublished astronomical spectra near 400 nm which contain broad interstellar absorption features consistent with the predictions from the laboratory matrix spectra, thus providing motivation for the recording of gas-phase electronic spectra of the internally cold triphenylene cation.