Unidentified Infrared Emission Bands Research Articles

Infrared Spectroscopy of Gas Phase Benzenium Ions: Protonated Benzene and Protonated Toluene, from 750 to 3400 cm–1

Gas phase C 6H 7 (+) and C 7H 9 (+) ions are studied with infrared photodissociation spectroscopy (IRPD) and the method of rare gas tagging. The ions are produced in a pulsed electric discharge supersonic expansion source from benzene or toluene precursors. We observe exclusively the formation of either the C 2 v benzenium ion (protonated benzene) or the para isomer of the toluenium ion (protonated toluene). The infrared spectral signatures associated with each ion are established between 750 and 3400 cm (-1). Comparing the gas phase spectrum of the benzenium ion to the spectrum obtained in a superacid matrix [ Perkampus, H. H.; Baumgarten, E. Angew. Chem. Int. Ed. 1964, 3, 776 ], we find that the C 2 v structure of the gas phase species is minimally affected by the matrix environment. An intense band near 1610 cm (-1) is observed for both ions and is indicative of the allylic pi-electron density associated with the six membered ring in these systems. This spectral signature, also observed for alkyl substituted benzenium ions and protonated naphthalene, compares favorably with the interstellar, unidentified infrared emission band near 6.2 microm (1613 cm (-1)).

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.

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.

Vibrational spectroscopy of interstellar molecules

In this paper we describe how infrared vibrational spectroscopy has been applied in our laboratory to three areas of interest to interstellar chemistry: the origin of the unidentified interstellar infrared emission bands, the depletion of the iron in the interstellar medium, and the fractionation of isotope-bearing molecules in space. In the first area, infrared absorption spectra of polycyclic aromatic hydrocarbons and their ions trapped in cryogenic rare gas matrices have been analyzed and shown to coincide well with the frequencies and relative intensities of the unidentified infrared emission bands. These bands have been observed from many sources in interstellar space, including reflection and planetary nebulae and H II regions, in parts of the Milky Way, as well as other galaxies. In the second area, a free electron laser, with tunable output in the infrared, has been utilized to record resonant multiphoton photodissociation vibrational spectra of various polycyclic aromatic hydrocarbons complexed with Fe+ in a Fourier transform ion cyclotron resonance mass spectrometer. Stable complexes found for Fe+ and a number of PAHs suggest that iron could be sequestered in such species and that they could function as carriers of the unidentified infrared emission bands. In the third area, the photo-induced 13C-isotopic scrambling in the C3 carbon cluster has been investigated by matrix isolation infrared spectroscopy. Fractionation is shown to arise from small zero-point energy differences in the C3 isotopomers. A proposed mechanism of isotopic scrambling involving the formation of cyclic C3 intermediates may be applicable to other species for which isotopic fractionation has also been observed in the ISM.

Mid‐ to Far‐Infrared Spectral Energy Distribution of the Diffuse Galactic Radiation

Mid-infrared (MIR; 12-60 μm) diffuse emission in the Galactic plane and the Carina Nebula is investigated relative to the far-infrared (FIR; ≥100 μm) emission. Observations show that the ratio of the 12 μm emission to the total FIR intensity is more or less constant in the Galactic plane but exhibits a slight decrease followed by an increase as the FIR color becomes bluer in the Carina Nebula. The constancy is compatible with predictions from models of stochastic heating of very small grains or infrared fluorescence of polycyclic aromatic hydrocarbons. The decrease can be attributed to a weakening of the unidentified infrared band emission in strong radiation fields. Contrarily, the ratio of the 25 and 60 μm emission to FIR intensity increases linearly with field strength, which is incompatible with the model predictions. The Carina Nebula data show a much bluer FIR color than the Galactic plane, whereas the ratio of MIR to FIR emission is in a range similar to that of the plane. We interpret these characteristics in terms of a superposition of emission from dust grains in various radiation field strengths. The linear increase can be accounted for by an increasing contribution from emission by dust grains in strong radiation environments, whereas the shift in FIR color between the Galactic plane and the Carina Nebula can be attributed to different contributions from grains in weak radiation fields. Other possibilities, such as the effect of multiphoton processes, variations in the incident radiation spectrum, and possible contributions from iron grains, have also been examined, but none can account for the observations consistently. Applications of the present model to external galaxies are also discussed.

Effects of dust scattering albedo and 2175-A bump on ultraviolet colours of normal disc galaxies

We discuss dust properties in the interstellar medium (ISM) of nearby normal galaxies, by comparing observations in the ultraviolet (UV) with simulations by a radiative transfer model. The observed UV colours of nearby galaxies show a reddening relative to their expected intrinsic colours. Some authors argued that the Milky Way dust cannot reproduce the reddening because of the prominent 2175 \AA absorption bump. Other authors proposed a reduction mechanism of the bump strength in an {\it attenuation law} derived from the ratio of the observed intensity to the intrinsic one through an age-selective attenuation (i.e., young stars are more attenuated selectively). We newly find that the wavelength dependence of the scattering albedo also has a strong effect on the UV colour; an albedo decreasing toward shorter wavelengths (except for the absorption bump range) produces a significant UV reddening. After comparing the observed UV colours of nearby normal galaxies with those expected from radiative transfer simulations assumed several dust models, we find two sorts of dust suitable for these galaxies: (1) dust with a bump and a smaller albedo for a shorter wavelength (except for the bump range), and (2) dust without any bump but with an almost constant albedo. If very small carbonaceous grains responsible for the common unidentified infrared emission band are also the bump carrier, the former dust is favorable. Finally, we derive mean attenuation laws of various dust models as a function of the UV attenuation, and derive some relations between the UV attenuation and observable/theoretical quantities.

Blue Blob in the Red Rectangle

ASTROPHYSICS The Red Rectangle nebula is an unusual protoplanetary nebula. Its central star, HD44179, is dying, and substantial mass is being blown off the star in a bipolar outflow, creating the rectangular shape and a large amount of dust. It is the brightest source of unidentified infrared band emission (UIR), and much of this comes from carbon compounds. Given that 20 to 30% of the Milky Way's infrared radiation is in the UIR and that carbon is an important component of stars, planets, and terrestrial life, it is desirable to identify the carbon species that account for the emissions. At shorter wavelengths, in the visible to ultraviolet, the Red Rectangle looks like a blue blob, produced by dust-scattered light from the central star. Vijh et al. obtained spectra at these wavelengths, and the peaks correspond to those in fluorescence spectra of electronic transitions in neutral polycyclic aromatic hydrocarbons (PAHs) with three to four rings, such as anthracene and pyrene. Thus, not only is the Red Rectangle producing a lot of PAH dust, but blue luminescence may be used to identify many carbon and silicon species that fluoresce at specific wavelengths. — LR Astrophys. J. , in press (astro-ph/0403522).

Discovery of Blue Luminescence in the Red Rectangle: Possible Fluorescence from Neutral Polycyclic Aromatic Hydrocarbon Molecules?

Here we report our discovery of a band of blue luminescence (BL) in the Red Rectangle (RR) nebula. This enigmatic proto-planetary nebula is also one of the brightest known sources of extended red emission as well as of unidentified infra-red (UIR) band emissions. The spectrum of this newly discovered BL is most likely fluorescence from small neutral polycyclic aromatic hydrocarbon (PAH) molecules. PAH molecules are thought to be widely present in many interstellar and circumstellar environments in our galaxy as well as in other galaxies, and are considered likely carriers of the UIR-band emission. However, no specific PAH molecule has yet been identified in a source outside the solar system, as the set of mid-infra-red emission features attributed to these molecules between the wavelengths of 3.3 micron and 16.4 micron is largely insensitive to molecular sizes. In contrast, near-UV/blue fluorescence of PAHs is more specific as to size, structure, and charge state of a PAH molecule. If the carriers of this near-UV/blue fluorescence are PAHs, they are most likely neutral PAH molecules consisting of 3-4 aromatic rings such as anthracene (C14H10) and pyrene (C16H10). These small PAHs would then be the largest molecules specifically identified in the interstellar medium.

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.

ISOCAM-CVF imaging of M 16

We present ISOCAM observations (5–17 μm) of the HST M 16 field, including the striking elephant trunk structures. The spectroscopic data have been obtained using the ISOCAM-CVF low-resolution mode, having a spectral resolution of ~40 () and a pixel size of 6″. The spectra show a combination of Unidentified Infrared Band (UIB) emission centred at 6.2, 7.7, 8.6, 11.3 and 16.4 μm, and atomic fine-structure lines of [ArII] 7.0 μm, [ArIII] 9.0 μm, [SIV] 10.5 μm, [NeII] 12.8 μm and [NeIII] 15.6 μm. The spectra were decomposed by fitting a combination of Lorentzian line profiles and a blackbody curve to the data to produce maps of the spatial distribution of the fine-structure and UIB emission features. These maps have been compared with VLA radio, 450 μm SCUBA maps, 12CO () maps, and broadband filters ISOCAM LW2 (5–8.5 μm) and LW3 (12–18 μm) observations. Decomposed spectral features have shown that the LW2 filter is dominated by UIB emission and the emission observed in the LW3 filter is a combination of Neon fine-structure lines and a steep continuum, with the continuum dominating. Using the photoionisation code CLOUDY we have produced theoretical values for the fine-structure ratio of [NeIII] 15.6 μm/[NeII] 12.8 μm, from which we have calculated the effective ionising flux impinging onto the tips of the pillars to be ~ photons cm-2 s-1, consistent with estimates made from VLA observations.

An ion beam reflectron/single-photon infrared emission spectrometer for the study of gas-phase polycyclic aromatic hydrocarbon ions: Testing proposed carriers of the unidentified infrared emission bands

We describe the design and performance of an electron-impact ion beam reflectron system coupled to our “single-photon infrared emission spectrometer” for measurement of infrared emission from excited gas-phase polycyclic aromatic hydrocarbon (PAH) cations. This experiment provides for direct comparison of laboratory infrared emission spectra of gaseous ionized PAHs with the “unidentified infrared emission bands,” the origin of which is still debated.

Single‐Photon Infrared Emission Spectroscopy of Gaseous Polycyclic Aromatic Hydrocarbon Cations: A Direct Test for Proposed Carriers of the Unidentified Infrared Emission Bands

We describe the measurement of infrared emission from excited gas-phase polycyclic aromatic hydrocarbons (PAHs) cations by an electron-impact ion beam reflectron system coupled to our ‘‘ single-photon infrared emission ’’ (SPIRE) spectrometer. This experiment provides for direct comparison of laboratory infrared emission spectra of gaseous ionized PAHs with the ‘‘ unidentified infrared emission bands ’’ (UIRs), the origin of which is still debated. We present results for the pyrene cation (C16H10 þ ) and the dehydrogenated pyrene cations (C16H8 þ and C16H6 þ ) showing general agreement between the corresponding SPIRE bands and UIR features, although it is not a detailed match. We also discuss possible contributions from the pyrene dimer cation ðC16H10Þ2 þ . Subject headings: dust: extinction — infrared: ISM — ISM: lines and bands — methods: laboratory — molecular data — techniques: spectroscopic

A Survey of Near Infrared Nebulosities around Luminous Young Stellar Objects: [ITAL]J[/ITAL], [ITAL]H[/ITAL], and [ITAL]K[/ITAL][arcmin] Imaging

We present J, H, and K' images of 45 IRAS sources in the recent list by Campbell, Persson, & Matthews, which have been presumed to be luminous young stellar objects (YSOs) because of their infrared colors and their large IRAS fluxes. These objects have been classified into four groups according to two dust features in the 3 μm band—the H2O ice absorption and the unidentified infrared band (UIB) emission. Our near-infrared images reveal that most of them (42/45) have nebulosity in at least one of the three bands. By examining the nebulosities around these YSOs, we find that (1) nebulae around the objects with the ice absorption can be explained by the scattered light of the central stars, (2) nebulae around the objects with the UIB emission cannot be explained by the scattered light alone—additional emission, most likely from very small grains, is necessary to explain the nebular colors and the surface brightness, and (3) the objects with neither the ice absorption nor the UIB emission tend to have faint or no nebulosity. We interpret the variation of nebular brightness and the color as evolutionary phases of circumstellar matter of intermediate- to high-mass YSOs.

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.

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.

The Galactic disc distribution of planetary nebulae with warm dust emission features — I

We investigate the Galactic disc distribution of a sample of planetary nebulae characterized in terms of their mid-infrared spectral features. The total number of Galactic disc PNe with 8–13 μm spectra is brought up to 74 with the inclusion of 24 new objects, the spectra of which we present for the first time. 54 PNe have clearly identified warm dust emission features, and form a sample that we use to construct the distribution of the C/O chemical balance in Galactic disc PNe. The dust emission features complement the information on the progenitor masses brought by the gas-phase N/O ratios: PNe with unidentified infrared emission bands have the highest N/O ratios, while PNe with the silicate signature have either very high N enrichment or close to none. We find a trend for a decreasing proportion of O-rich PNe towards the third and fourth Galactic quadrants. Two independent distance scales confirm that the proportion of O-rich PNe decreases from per cent inside the solar circle to per cent outside. PNe with warm dust are also the youngest. PNe with no warm dust are uniformly distributed in C/O and N/O ratios, and do not appear to be confined to They also have higher 6-cm fluxes, as expected from more evolved PNe. We show that the IRAS fluxes are a good representation of the bolometric flux for compact and IR-bright PNe, which are probably optically thick. Selection of objects with should probe a good portion of the Galactic disc for these young, dense and compact nebulae, and the dominant selection effects are rooted in the PN catalogues.

A New Mid-Infrared Diagnostic between AGN and Starbursts

We present a new set of diagnostics which allow us to trace and classify in a statistical manner the mid-IR emission produced by active galactic nuclei (AGN) and star-forming regions. We construct a diagram based on the strength of the unidentified infrared band (UIB)at 6.2 μm, and the intensity of the continuum at short (6 μm) and long wavelengths (15 μm). We interpret the integrated mid-IR emission in late-type galaxies as resulting from three individual contributions coming from HII regions, diffuse/photodissociation regions (PDRs), and AGN. Based on this assumption, our diagnostic diagram provides a quantitative estimate of the AGN and starburst contribution to an observed mid-IR spectrum. We show that UIB emission is very faint or absent in regions harbouring intense and hard radiation fields as in the case of AGN or ‘pure’ HII starburst regions where UIB carriers can be destroyed by photodissociation. However, contrary to starburst spectra, typical AGN spectra present a strong hot continuum below 9 μm originating from hot dust heated by the AGN radiation field. An extrapolation of this diagnostic towards other mid-IR observations should improve our knowledge of the AGN/starburst connection.

Peripherally Hydrogenated Neutral Polycyclic Aromatic Hydrocarbons as Carriers of the 3 Micron Interstellar Infrared Emission Complex: Results from Single‐Photon Infrared Emission Spectroscopy

Infrared emission spectra of five gas-phase UV laser-excited polycyclic aromatic hydrocarbons (PAHs) containing aliphatic hydrogens are compared with the main 3.3 microns and associated interstellar unidentified infrared emission bands (UIRs). We show that neutral PAHs can account for the majority of the 3 microns emission complex while making little contribution to the other UIR bands; peripherally hydrogenated PAHs produce a better match to astrophysical data than do those containing methyl side groups; 3.4 microns plateau emission is shown to be a general spectral feature of vibrationally excited PAHs containing aliphatic hydrogens, especially those containing methyl groups; and finally, hot-band and overtone emissions arising from aromatic C-H vibrations are not observed in laboratory emission spectra, and therefore, in contrast to current assignments, are not expected to be observed in the UIRs.

Unidentified Infrared Emission Bands in the Diffuse Interstellar Medium

Using the mid-infrared spectrometer on board the Infrared Telescope in Space and the low-resolution grating spectrometer (PHT-S) on board the Infrared Space Observatory, we obtained 820 mid-infrared (5-12 μm) spectra of the diffuse interstellar medium (DIM) in the Galactic center, W51, and Carina Nebula regions. These spectra indicate that the emission is dominated by the unidentified infrared (UIR) emission bands at 6.2, 7.7, 8.6, and 11.2 μm. The relative band intensities (6.2/7.7 μm, 8.6/7.7 μm, and 11.2/7.7 μm) were derived from these spectra, and no systematic variation in these ratios was found in our observed regions, in spite of the fact that the incident radiation intensity differs by a factor of 1500. Comparing our results with the polycyclic aromatic hydrocarbons (PAHs) model for the UIR band carriers, we find that PAHs in the DIM have no systematic variation in their size distribution, their degree of dehydrogenation is independent of the strength of UV radiation field, and they are mostly ionized. The finding that PAHs in the DIM with low UV radiation field strength are mostly ionized is incompatible with past theoretical studies, in which a large fraction of neutral PAHs is predicted in this kind of environment. A plausible resolution of this discrepancy is that the recombination coefficients for electron and large PAH positive ion are by at least an order of magnitude less than those adopted in past theoretical studies. Because of the very low population of neutral state molecules, photoelectric emission from interstellar PAHs is probably not the dominant source of heating of the diffuse interstellar gas. The present results imply constant physical and chemical properties of the carriers of the UIR emission bands in the DIM covering the central and disk regions of the Galaxy, which could help in the identification of the carriers.