Unidentified Infrared Emission Research Articles

Laboratory Spectroscopy of Protonated PAH Molecules Relevant For Interstellar Chemistry

In this contribution, we summarize the recent progress made in recording laboratory infrared (IR) spectra of protonated polycyclic aromatic hydrocarbon molecules (H + PAH) in the gas phase. The IR spectra of a large variety of H + PAH species ranging from benzene to coronene have been obtained by various variants of photodissociation spectroscopy. The employed techniques include single-photon IR photodissociation (IRPD) of tagged H + PAH ions and IR multiple-photon dissociation (IRMPD) of bare H + PAH ions. The comparison of the laboratory IR spectra with astronomical spectra supports the hypothesis that H + PAH ions are possible carriers of the unidentified IR emission (UIR) bands. Moreover, the spectra provide detailed information about the geometric and electronic structure as well as the chemical reactivity and stability of these fundamental hydrocarbon ions.

Computational IR Spectroscopy for PAHs: from the early years to the present status

In the long story of interstellar PAHs, computations have played and are still playing a fundamental role in connection with experiments and observations. From the very first calculations of the IR spectra of small PAHs in the late eighties to the more recent ones, every aspect of the research linked to the PAH hypothesis has evolved dramatically: the size and the variety of the molecules considered, the techniques used, the precision of the astronomical observations ... The initial landscape has completely changed though the quest is still the same, that is to correlate the so-called UIR bands spectra ubiquitous in the ISM (Inter Stellar Medium) with a chemical family of molecules, the PAHs. An historical review of the 25 years of this quest is presented here, focusing on the computational part.

Detection of unidentified infrared bands in a Hαfilament in the dwarf galaxy NGC 1569 with AKARI

We performed observations of NGC1569 for 6 infrared bands (3.2, 4.1, 7, 11, 15, and 24 micron) with the Infrared Camera (IRC) onboard AKARI. Near- to mid-infrared (2--13 micron) spectroscopy of a Halpha filament was also carried out with the IRC. The extended structure associated with a Halpha filament appears bright at 7 micron, suggesting that the filament is bright at the UIR band emission. Follow-up spectroscopic observations with the IRC confirm the presence of 6.2, 7.7, and 11.3 micron emission in the filament. The filament spectrum exhibits strong 11.3 micron UIR band emission relative to the 7.7 micron band compared to the galaxy disk observed with the Infrared Spectrograph on Spitzer. The near-infrared spectrum also suggests the presence of excess continuum emission in 2.5--5 micron in the filament. The Halpha filament is thought to have been formed by the galactic outflow originating from the star-formation activity in the disk of NGC1569. The destruction timescale of the UIR band carriers in the outflow is estimated to be much shorter (~ 1.3 x 10^3 yr) than the timescale of the outflow (~ 5.3 Myr). Thus it is unlikely that the band carriers survive the outflow environment. Alternatively, we suggest that the band carriers in the filaments may be produced by the fragmentation of large carbonaceous grains in shocks, which produces the Halpha emission. The NIR excess continuum emission cannot be accounted for by free-free emission alone and a hot dust contribution may be needed, although the free-free emission intensity estimated from HI recombination lines has a large uncertainty.

Infrared spectra of protonated polycyclic aromatic hydrocarbon molecules: Azulene

The infrared (IR) spectrum of protonated azulene (AzuH(+), C(10)H(9)(+)) has been measured in the fingerprint range (600-1800 cm(-1)) by means of IR multiple photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer equipped with an electrospray ionization source using a free electron laser. The potential energy surface of AzuH(+) has been characterized at the B3LYP/6-311G(**) level in order to determine the global and local minima and the corresponding transition states for interconversion. The energies of the local and global minima, the dissociation energies for the lowest-energy fragmentation pathways, and the proton affinity have been evaluated at the CBS-QB3 level. Comparison with calculated linear IR absorption spectra supports the assignment of the IRMPD spectrum to C4-protonated AzuH(+), the most stable of the six distinguishable C-protonated AzuH(+) isomers. Comparison between Azu and C4-AzuH(+) reveals the effects of protonation on the geometry, vibrational properties, and the charge distribution of these fundamental aromatic molecules. Calculations at the MP2 level indicate that this technique is not suitable to predict reliable IR spectra for this type of carbocations even for relatively large basis sets. The IRMPD spectrum of protonated azulene is compared to that of isomeric protonated naphthalene and to an astronomical spectrum of the unidentified IR emission bands.

INFRARED SPECTRA OF ISOLATED PROTONATED POLYCYCLIC AROMATIC HYDROCARBON MOLECULES

Gas-phase infrared (IR) spectra of larger protonated polycyclic aromatic hydrocarbon molecules, H+PAH, have been recorded for the first time. The ions are generated by electrospray ionization and spectroscopically assayed by IR multiple-photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer using a free electron laser. IRMPD spectra of protonated anthracene, tetracene, pentacene, and coronene are presented and compared to calculated IR spectra. Comparison of the laboratory IR spectra to an astronomical spectrum of the unidentified IR emission (UIR) bands obtained in a highly ionized region of the interstellar medium provides for the first time compelling spectroscopic support for the recent hypothesis that H+PAHs contribute as carriers of the UIR bands.

SMALL CARBONACEOUS MOLECULES, ETHYLENE OXIDE (c-C2H4O) AND CYCLOPROPENYLIDENE (c-C3H2): SOURCES OF THE UNIDENTIFIED INFRARED BANDS?

We suggest that small carbonaceous molecules (SCMs) may be the sources of the unidentified infrared bands (UIRs) and the underlying continuum. We show that the IR spectroscopy of ethylene oxide (EO, c-C2H4O) and cyclopropenylidene (CP, c-C3H2) closely correlates with the major UIR bands at 3.3, 6.2, 7.7, 8.6, and 11.2 μm, the often seen strong bands at 12.7 and 16.4 μm, as well as many minor features. The differences in band locations and shapes between laboratory EO absorption spectra and astrophysical UIR emission spectra are attributed to vibrational anharmonicity, Fermi resonance splitting of nearly degenerate vibration levels, and rotational envelope narrowing due to the low temperatures in space. The excitation mechanism is absorption of UV radiation, primarily Lyα, by SCMs. Photon trapping for this very optically thick transition enhances the absorption by several orders of magnitude. Our abundance analysis for NGC 7027 reveals that the SCM abundance, relative to H2, is ~3 × 10–9 which compares well to radio measurements of the CP abundance range of ~10–9-10–7. The origin of the UIR continuum is discussed in terms of emission from vibrationally and rotationally hot SCM UV photodissociation products and UV excitation of rotationally hot SCM species. Radio lines of CP have been seen in numerous astronomical objects, most displaying the UIR bands. EO is also seen, but in fewer objects, none displaying the UIR bands. We theorize that in UIR objects, EO is formed on, and primarily resides on, carbonaceous grains, precluding radio detection of rotational lines. We suggest laboratory experiments, astronomical observations, and theoretical investigations to further evaluate the SCM mechanism for the UIR bands and continuum.

Organic compounds in galaxies

AbstractThe unidentified infrared (UIR) emission bands in the near- to mid-infrared are thought to originate from organic compounds in the interstellar medium. Recent space observations with Spitzer and AKARI have clearly revealed that the UIR bands are commonly seen in external galaxies, including elliptical galaxies, except for very metal-poor dwarf galaxies. They are also detected in extended structures of galaxies, such as extra-planar components and filaments produced by outflows, suggesting that the band carriers are ubiquitous organic compounds in galaxies. Since the UIR bands are prominent features in the infrared spectrum of galaxies and are linked to the star-formation activity, it is highly important to understand the nature, formation, processing, and destruction of the UIR band carriers in galaxies. While there is no systematic variation detected in the UIR spectrum in normal galaxies, significantly low values are derived for the ratio of the 7.7 μm to 11.2 μm bands in elliptical galaxies as well as in galaxies with low-luminosity AGNs compared to normal star-forming galaxies. Relatively low band ratios are also seen in the UIR band spectrum of extended structures in galaxies. If the same mechanism leads to the low band ratio, it would provide important information on the band carrier properties. It should also be noted that the band carriers are believed to be destroyed in a short time scale in environments where low band ratios are detected. The survival and supply processes in these environments are a key to understand the nature of the band carriers.

Properties of polycyclic aromatic hydrocarbons in the star forming environment in nearby galaxies

AbstractWe have carried out the mid-infrared slit spectroscopic observations of sources in the LMC and in NGC 6946 with AKARI/IRC. We investigate the properties of the UIR bands in terms of the star forming activities. We find systematically larger ratios of UIR bands in 6–9 μm to 11.2 μm band in active star forming regions than in the quiet regions. This behavior is consistent with the photo-ionization model of PAHs. Our results suggest that the ratios of UIR bands in 6–9 μm to 11.2 μm band can be used as more efficient and vigorous tools to measure the extent of on-going star formation in remote galaxies rather than just the presence or absence of the features themselves.

Vibrational spectroscopy of neutral complexes of Fe and polycyclic aromatic hydrocarbons

Infrared absorption spectra of neutral complexes of iron with benzene, naphthalene, fluorene, pyrene, and coronene in solid Ar at 12 K have been obtained. Supporting calculations of the equilibrium geometries, stabilities, and harmonic vibrational frequencies of these complexes have been carried out using density functional theory (MPW1PW91/6-31+G(d,p) method) using a modified Perdew–Wang exchange and correlation functional/basis set. Our results indicate that the spin multiplicities of the complexes’ electronic ground states are triplets. The calculations show that the iron atom is situated over the six-membered carbon ring of the polycyclic aromatic hydrocarbon (PAH) ligand. Calculated dissociation energies ( D 0) range from 0.52 eV for Fe(coronene) to 2.06 eV for Fe(fluorene). All are substantially less tightly bound than their cationic counterparts. Cationic Fe(PAH) complexes are therefore thought to be more plausible candidates for the carriers of the unidentified interstellar infrared (UIR) emission bands.

A cold globule with a Class 0/I embedded source

Infrared observations of the dark cloud DC 303.8-14.2 and the embedded point source IRAS 13036-7644 have been made with the ISOPHOT instrument aboard ISO at 7.7 μm, 60 μm, 100 μm and 200 μm. We have compared the properties of dust in DC 303.8-14.2 and the Thumbprint Nebula (TPN), a morphologically similar globule but without star formation. The dust temperature at the center of DC 303.8-14.2 has a minimum of 14.6 ± 1 K, similar to that in the TPN. A comparison of far-infrared radial optical depth distributions between these clouds at angular scales of ∼1.5'-3' shows no difference. The observations suggest that there is a bright rim of 7,7 μm emission that peaks just outside the,optical bright rim of the cloud, indicating a halo of very small dust particles or PAHs around the cloud. The bolometric luminosity of the IRAS source is estimated to be about 1.0 L ○. . The heating of the cloud by the IRAS source is negligible; the thermal structure of the cloud is dominated by external heating. We have studied the evolutionary status of the IRAS source and find it to be located between Class 0 and Class I, at a late accretion phase. The source was detected at 7.7 μm and we suggest that this emission is due to the 7.7 μm UIR band. We also give a compilation of small molecular clouds whose extended FIR emission has been mapped with ISO.

Spatially and spectrally resolved 10 μm emission in Herbig Ae/Be stars

We present new mid-infrared spectroscopy of the emission from warm circumstellar dust grains in the Herbig Ae stars HD 100546, HD 97048 and HD 104237, with a spatial resolution of ≈09. We find that the emission in the UIR bands at 8.6, 11.3 and (HD 97048 only) 12.7 μm is extended in the first two sources. The continuum emission is resolved in HD 97048 and possibly in HD 100546. HD 104237 is not spatially resolved in our observations. We find that the UIR emission in HD 100546 and HD 97048 is extended on a scale of (several) 100 AU, corresponding to the outer disk scale in flaring disk models. Small carbonaceous particles are the dominant source of opacity in the HD 97048 disk.

Sub-arcsecond imaging and spectroscopic observations of the Red Rectangle in theN-band

We carried out imaging and long-slit spectroscopic observations of the Red Rectangle in the N-band using the 8.2-m SUBARU telescope. The resulting images reveal a central bright core and an extended nebula. The central core is slightly extended in the N-band. The core radius is estimated to be ~, which corresponds to 60 AU. The core spectrum is well modeled by blackbody emission with a temperature of 358 K. The estimated size and temperature are in good agreement with the predictions of recent dusty torus models. In the north/south nebula, unidentified infrared (UIR) emission features at 8.6 μm and 11.2 μm are prominently observed. At radii > they completely dominate the N-band spectra, suggesting that the nebula seen at N-band wavelengths is primarily brightened by the UIR-band emission, not by scattered light. We also detected shifts in the UIR-feature peaks with increasing distance from the center. These may be attributed to isotopic shifts due to the presence of in polycyclic aromatic hydrocarbons (PAH) like particles. In addition to the UIR emission, a broad feature at 11.27 μm is seen at a distance of 0.3´´ on all sides of the nebula. We attribute this to crystalline silicate olivine grains around the central torus.

Evolution of the 3.3- m emission feature in the Red Rectangle

Medium-resolution infrared spectroscopic observations of the biconical Red Rectangle nebula in the region of the 3.3-μm unidentified infrared (UIR) emission band are reported. The data were recorded at UKIRT using CGS4 and have allowed the peak wavelength, width, profile and intensity of the 3.3-μm feature to be investigated as a function of offset from the central star HD 44179. Analysis of the profile of the feature along the north-western bicone interface shows an evolution from Type 2 to Type 1 in the classification of Tokunaga et al. The 3.3-μm band recorded on-star shows a close fit to a Lorentzian profile, possibly suggesting a single class of carriers. Subtraction of the Lorentzian fit to the on-star spectrum from the spectrum at each offset reveals a new ‘3.28’-μm emission feature that grows in intensity relative to the main 3.3-μm band as a function of distance from the central star. The 3.28-μm emission band at large offset appears to correspond well in wavelength and FWHM with an absorption feature seen towards the Galactic Centre.

The Infrared Emission Features in the Spectrum of the Wolf-Rayet star WR 48[CLC]a[/CLC

We present the first detection of unidentified infrared (UIR) emission features at ~6.4 and 7.9 μm in the spectrum of the dusty WC8 Wolf-Rayet star WR 48a. Based on the H-deficient nature of WC stars, we attribute the emission features to large carbonaceous molecules or amorphous carbon dust grains in the circumstellar environment of WR 48a. The 6.4 μm feature resembles the emission feature seen toward H-deficient planetary nebulae (PNe), while the 7.9 μm profile resembles that of some PNe with H-deficient WC10 central stars. These similarities point toward a similar origin of the dust in these H-deficient environments and highlights the apparent sensitivity of the UIR bands to physical conditions. In the case of WR 48a and the [WC10] PNe, shock processing may play a major role in dust formation.

Near‐Infrared and CO (J = 1–0) Observations of Photodissociation Regions in M17

We have carried out near-infrared mapping observations of photodissociation regions in M17 with the Wide Field Cryogenic Telescope and CO (J = 1-0) observations in three isotope lines with the NANTEN telescope. The observations covered an area of 20' × 20' with a spatial resolution of 56 for near-infrared wavelengths and with a half-power beamwidth of 27 for millimeter wavelengths. We detected 38 sources brighter than 7 mag at 3.67 μm (Ln band), five of which show signs of young stellar objects. We have detected two emission bars (the N bar and the S bar) in all four near-infrared bands (J, K, Ln, and 3.3 μm). Their spatial distributions differ considerably from band to band, and we have compared them with the radio continuum, the mid-infrared data, and the CO molecular line emission. The different brightness and spectral energy distributions at near-infrared wavelengths can be well explained by emission from hot dust and ionized gas together with obscuration by local cold dust with a steep gradient from north to south. In the N bar, the free-free emission from ionized gas dominates at shorter wavelengths (J and K) and there is little extinction, whereas in the S bar, the free-free emission is attenuated at shorter wavelengths by the heavy local extinction. In both the N and S bars, the thermal emission from hot dust at around 1000 K dominates in the Ln band. The 3.3 μm unidentified infrared (UIR) emission delineates photodissociation regions between the H II regions and the surrounding molecular clouds. The UIR intensity decreases exponentially from the UIR peak toward the molecular clouds, with scale lengths of 88'' and 100'', or 0.9 and 1.0 pc, at the N and the S bars, respectively. Far-ultraviolet photons, which excite UIR emission, penetrate into the molecular clouds for ~1 pc, in the nearly edge-on geometry. The 12CO contours are elongated in the direction northwest-southeast, while the C18O contours are round. Far-ultraviolet photons erode the tenuous portions (as seen in 12CO) of the surface of the cloud and penetrate deeply toward the denser inside, forming complex structures in the photodissociation regions bordering the molecular cloud.

The infrared signatures of non-regular PAHs

In a first approach to the spectral signatures of an irregular carbonaceous matter, we report a theoretical investigation of the infrared spectra of non-regular conjugated hydrocarbons. The series under consideration consists of three-cycle PAHs. Anthracene and phenanthrene are taken as references for regular PAHs and dibenzocyclobutadiene (biphenylene), dibenzocyclopentadienyl (fluorenyl), dibenzocycloheptatrienyl (heptafluorenyl) as models for irregular PAHs. These irregular conjugated hydrocarbons consist of four-, five- and seven- (instead of six) membered rings respectively sandwiched between two benzene entities while the regular ones are built with only six-membered rings. The IR spectra of the neutral, singly and doubly ionized species are compared. It is found that the non-regular compounds present the same general behaviour as regular ones at each level of ionization. From an astrophysical point of view, none of the differences is found big enough to be relevant for selective observation, which implies that all sorts of PAHs including irregular structures may be present and contribute anonymously to the UIR bands.

ISOPHOT observations of the UIR bands in the diffuse interstellar medium

Strong and broad emission bands commonly attributed to polycyclic hydrocarbons dominate the mid-infrared spectrum of interstellar matter. Early photo- and spectrometry by the ISO and IRTS missions has proved that these bands are widespread in the diffuse interstellar medium. We have sampled the galactic distribution of unidentified infrared band emission in the diffuse ISM component. The remarkable insensitivity of the shape of the UIR band spectrum is seen in this sample; little variation with line strength or galactic position is seen suggesting either a homogeneous history of the UIR band carriers within the Milky Way, convergent evolution of grains suppressing any differences seen in younger populations or an emission mechanism highly insensitive to particle type and physical conditions.

Single photon infrared emission spectroscopy of the gas phase pyrene cation: support for a polycyclic aromatic hydrocarbon origin of the unidentified infrared emission bands.

We report the first observation of infrared emission from a gaseous ionic polycyclic aromatic hydrocarbon (PAH), the pyrene cation, over the range of wavelengths spanned by the ubiquitous interstellar unidentified infrared emission bands (UIRs). The complete set of pyrene cation IR emissions is observed, with relative intensities consistent with astrophysical observations, supporting the proposal that ionized PAHs are major contributors to the UIR bands. Additionally, unidentified features possibly arising from dehydrogenated PAH species are noted in the spectrum.

The CH out-of-plane bending modes of PAH molecules in astrophysical environments

We present 10 15 m spectra of as ample of Hii regions, YSOs and evolved stars that show strong unidentied infrared emission features, obtained with the ISO/SWS spectrograph on-board ISO. These spectra reveal a plethora of emission features with bands at 11.0, 11.2, 12.0, 12.7, 13.5 and 14.2 m. These features are observed to vary considerably in relative strength to each-other from source to source. In particular, the 10{15m spectra of the evolved stars are dominated by the 11.2 m band while for H ii regions the 12.7 is typically as strong as the 11.2 m band. Analysing the ISO data we nd a good correlation between the 11.2 mb and and the 3.3 m band, and between the 12.7 ma nd the 6.2m band. There is also a correlation between the ratio of the UIR bands to the total dust emission and the 12.7 over 11.2 m ratio. Bands in the 10{15 ms pectral region are due to CH out of plane (OOP) bending modes of polycyclic aromatic hydrocarbons (PAHs). We summarise existing laboratory data and theoretical quantum chemical calculations of these modes for neutral and cationic PAHs. Due to mode coupling, the exact peak position of these bands depends on the number of adjacent CH groups and hence the observed interstellar 10 15 m spectra can be used to determine the molecular structure of the interstellar PAHs emitting in the dierent regions. We conclude that evolved stars predominantly inject compact 100 200 C-atom PAHs into the ISM where they are subsequently processed, resulting in more open and uneven PAH structures.

Hydrogenation of polycyclic aromatic hydrocarbons as a factor affecting the cosmic 6.2 micron emission band

While many of the characteristics of the cosmic unidentified infrared (UIR) emission bands observed for interstellar and circumstellar sources within the Milky Way and other galaxies, can be best attributed to vibrational modes of the variants of the molecular family known as polycyclic aromatic hydrocarbons (PAH), there are open questions that need to be resolved. Among them is the observed strength of the 6.2 micron (1600 cm −1) band relative to other strong bands, and the generally low strength for measurements in the laboratory of the 1600 cm −1 skeletal vibration band of many specific neutral PAH molecules. Also, experiments involving laser excitation of some gas phase neutral PAH species while producing long lifetime state emission in the 3.3 micron (3000 cm −1) spectral region, do not result in significant 6.2 micron (1600 cm −1) emission. A potentially important variant of the neutral PAH species, namely hydrogenated-PAH (H N–PAH) which exhibit intriguing spectral correlation with interstellar and circumstellar infrared emission and the 2175 Å extinction feature, may be a factor affecting the strength of 6.2 micron emission. These species are hybrids of aromatic and cycloalkane structures. Laboratory infrared absorption spectroscopy augmented by density function theory (DFT) computations of selected partially hydrogenated-PAH molecules, demonstrates enhanced 6.2 micron (1600 cm −1) region skeletal vibration mode strength for these molecules relative to the normal PAH form. This along with other factors such as ionization or the incorporation of nitrogen or oxygen atoms could be a reason for the strength of the cosmic 6.2 micron (1600 cm −1) feature.