Interstellar Absorption Bands Research Articles

Interstellar Formation of Nitrogen Heteroaromatics [Indole, C8H7N; Pyrrole, C4H5N; Aniline, C6H5NH2]: Key Precursors to Amino Acids and Nucleobases.

Nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) are not only fundamental building blocks in the prebiotic synthesis of vital biomolecules such as amino acids and nucleobases of DNA and RNA but also a potential source of the prominent unidentified 6.2 μm interstellar absorption band. Although NPAHs have been detected in meteorites and are believed to be ubiquitous in the universe, their formation mechanisms in deep space have remained largely elusive. Here, we report the first bottom-up formation pathways to the simplest prototype of NPAHs, indole (C8H7N), along with its building blocks pyrrole (C4H5N) and aniline (C6H5NH2) in low-temperature model interstellar ices composed of acetylene (C2H2) and ammonia (NH3). Utilizing the isomer-selective techniques of resonance-enhanced multiphoton ionization and tunable vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry, indole, pyrrole, and aniline were identified in the gas phase, suggesting that they are promising candidates for future astronomical searches in star-forming regions. Our laboratory experiments utilizing infrared spectroscopy and mass spectrometry in tandem with electronic structure calculations reveal critical insights into the reaction pathways toward NPAHs and their precursors, thus advancing our fundamental understanding of the interstellar formation of aromatic proteinogenic amino acids and nucleobases, key classes of molecules central to the Origins of Life.

EVALUATION OF THE POTENTIAL NEUROACTIVITY IN THE BRAIN NERVE TERMINALS OF THE C60 FULLERENE PLANETARY DUST COMPONENT

A study of the infrared spectrum of the environment of the planetary nebula Tc 1 revealed the radiation of cold and neutral fullerenes C60 and C70. The results of the analysis of infrared spectra obtained using the Hubble space telescope conclusively proved the existence of C60+ fullerene in the interstellar medium. These large carbon-containing molecules can form and exist in the interstellar medium and are candidates to explain many diffuse interstellar absorption bands. In this study, the potential neuroactivity of the C60 fullerene as a planetary dust component was assessed in the isolated rat brain nerve terminals. It was shown that C60 fullerene in the unirradiated state at concentrations of 0.05—0.25 mg/ml did not change the extracellular levels of excitatory neurotransmitter L-[ 14C]glutamate and inhibitory neurotransmitter [ 3H]GABA in the preparations of rat brain nerve terminals. An increase in fullerene C60 concentrations up to 0.5 and 1.00 mg/ml was accompanied by an increase in the extracellular levels of L-[ 14C]glutamate and [3 H]GABA in the preparations of nerve terminals. Therefore, fullerene C60 did not cause signs of acute neurotoxicity in the brain nerve terminals within the concentration range of 0.05—0.25 mg/ml. However, given that C60 undergoes photooxidation, it can be expected that it may acquire neurotoxic properties in situ.

Galactic extinction laws – II. Hidden in plain sight, a new interstellar absorption band at 7700 Å broader than any known DIB

ABSTRACT We have detected a broad interstellar absorption band centred close to 7700 Å and with a full width at half-maximum (FWHM) of 176.6 ± 3.9 Å. This is the first such absorption band detected in the optical range and is significantly wider than the numerous diffuse interstellar bands (DIBs). It remained undiscovered until now because it is partially hidden behind the A telluric band produced by O2. The band was discovered using STIS@HST spectra and later detected in a large sample of stars of diverse type (OB stars, BA supergiants, red giants), using further STIS and ground-based spectroscopy. The EW of the band is measured and compared with our extinction and K i λλ7667.021, 7701.093 measurements for the same sample. The carrier is ubiquitous in the diffuse and translucent Galactic interstellar medium (ISM) but is depleted in the environment around OB stars. In particular, it appears to be absent or nearly so in sightlines rich in molecular carbon. This behaviour is similar to that of the σ-type DIBs, which originate in the low/intermediate-density UV-exposed ISM but are depleted in the high-density UV-shielded molecular clouds. We also present an update on our previous work on the relationship between E(4405–5495) and R5495 and incorporate our results into a general model of the ISM.

Diffuse Interstellar Bands as Absorption by Frozen Oriented Gas

The nature of interstellar absorption bands is examined. A new mechanism for their formation is proposed, specifically resonance absorption of luminescence by frozen organics in grains with identical chemical composition. The possibility of interstellar quasiline absorption is considered. A conceptual scheme for an organic optical filter is proposed. The proposed theory is used to identify a number of absorption bands and the results of a comparative analysis are presented.

Interstellar carbon dust

Carbon is the fourth most abundant element in the Universe after hydrogen, helium and oxygen. Synthesized at the late evolutionary stage of stars, carbon is ejected from stars through mass loss of asymptotic giant branch stars and explosion of supernovae and then enters the interstellar medium in the form of ions, atoms, molecules and solid dust particles. Carbon-containing complex organic molecules are essential for the origins of life, and carbonaceous dust is a major component of interstellar grains whose infrared absorption and emission spectra provide a powerful diagnosis of the physical and chemical conditions of astrophysical regions where the dust is found. Carbonaceous dust materials such as graphite, nanodiamonds, polycyclic aromatic hydrocarbon molecules, fullerene, and hydrogenated amorphous carbon are ubiquitously present in the interstellar space and are the most promising candidate carriers of the unidentified 2175 A extinction bump, the unidentified infrared emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 µm, and the mysterious diffuse interstellar absorption bands. Here we focus on the observational properties of interstellar carbonaceous grains and their physical and chemical characteristics. Also discussed are graphene and carbon nanotubes which may be present in the interstellar space.

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.

The λ6614 diffuse interstellar absorption band: evidence for internal excitation of the carrier

An analysis of absorption profiles of the $\lambda$6614 diffuse interstellar band recorded along the lines-of-sight towards HD 179406 (20 Aql) and HD 147889 is described. The difference in band shape is attributed to the degree of internal excitation of the carrier, which is principally due to vibrational hot bands although an electronic component may also be present. The results are discussed with respect to other models of diffuse band spectral line shape.

XUV excitation followed by ultrafast non-adiabatic relaxation in PAH molecules as a femto-astrochemistry experiment.

Highly excited molecular species are at play in the chemistry of interstellar media and are involved in the creation of radiation damage in a biological tissue. Recently developed ultrashort extreme ultraviolet light sources offer the high excitation energies and ultrafast time-resolution required for probing the dynamics of highly excited molecular states on femtosecond (fs) (1 fs=10−15s) and even attosecond (as) (1 as=10−18 s) timescales. Here we show that polycyclic aromatic hydrocarbons (PAHs) undergo ultrafast relaxation on a few tens of femtoseconds timescales, involving an interplay between the electronic and vibrational degrees of freedom. Our work reveals a general property of excited radical PAHs that can help to elucidate the assignment of diffuse interstellar absorption bands in astrochemistry, and provides a benchmark for the manner in which coupled electronic and nuclear dynamics determines reaction pathways in large molecules following extreme ultraviolet excitation.

Extracting interstellar diffuse absorption bands from cool star spectra

Interstellar diffuse bands are usually extracted from hot star spectra because they are characterized by smooth continua. It introduces a strong limitation on the number of available targets, and reduces potential studies of the IS matter and the use of absorptions for cloud mapping. We have developed a new automatic fitting method appropriate to interstellar absorptions in spectra of cool stars that possess stellar atmospheric parameters. We applied this method to the extraction of three DIBs in high resolution VLT FLAMES/GIRAFFE spectra of red clump stars from the bulge. By combining all stellar synthetic spectra, HITRAN-LBLRTM atmospheric transmission spectra and diffuse band empirical absorption profiles, we determine the 6196, 6204, and 6284 A DIB strength toward the 219 target stars and discuss the sources of uncertainties. In order to test the sensitivity of the DIB extraction, we intercompare the three results and compare the DIB equivalent widths with the reddening derived from an independent extinction map based on OGLE photometric data. Most stellar spectra could be well reproduced by the composite stellar, atmospheric and interstellar models. Measurement uncertainties on the EWs are smaller for the broad and strong 6284 A DIB, and are of the order of 10-15%. Uncertainties on the two narrow and weaker DIBs are larger, as expected, and found to be highly variable from one target to the other. They strongly depend on the radial velocity of the star . DIB-DIB correlations among the three bands demonstrate that a meaningful signal is extracted. For the 6284 and 6204 A DIBs, the star-to-star variability of the equivalent width (EW) also reflects features of the OGLE extinction map...

Spectroscopic observations of the recurrent nova CI Aquilae during the 2000 outburst

High- and low-resolution optical spectra of CI Aql were obtained during the outburst in 2000. Multiple absorption components of H I and FeII lines were detected at the early decline stage. Their radial velocities were roughly −2500kms −1 , −2200kms −1 , −1700kms −1 ,a nd−1400kms −1 , among which only the last components were likely still accelerating during the early decline stage. Prominent emission lines of [OIII] and [NII] appeared about one month after light maximum. The duration of the nebular stage, however, was only one month and a few weeks. The ejected gas shells seem to have started to shrink about 70 days after light maximum. The amount of interstellar extinction is estimated to be E(B − V) = 0.92 ± 0.15 from the equivalent widths of the diffuse interstellar absorption bands. The helium abundance in the ejecta is estimated to be N(He) = 0.19 ± 0.05 and the mass of the ejecta to be about 2 × 10 −6 M� . This object has been classified as a U Sco type recurrent nova, but its spectral evolution during the outburst resembled those of T Pyx type recurrent novae. It is doubtful whether the peak of mV 9 mag on 2000 May 5 was the true light maximum or an earlier brighter peak had been overlooked, because the spectral data suggest that the ejections of gas shells occurred prior to the discovery of the outburst on 2000 April 28.

C84: A Prototype of Larger Fullerenes. Laboratory Spectroscopy and Astronomical Relevance

We present for the first time the UV/Vis spectrum of the neutral and ionized fullerene C84 isolated in a neon matrix at low temperature. After ionization with high energy (10.2 eV) UV photons, we are able to identify a new spectroscopic absorption band at 718.2 nm that we tentatively attribute to the C84 + ion. We compare the optical spectrum to the diffuse interstellar absorption bands (DIBs) and find eight possible DIBs that fall within the expected shift induced by the solid matrix (± 5 nm) of the assigned C84 + absorption band at 713.6, 713.8, 715.4, 716.1, 718.0, 722.3, 722.4, and 722.8 nm. The astronomical C84 + column density that is required to account for the observed strength of these DIBs is calculated assuming the oscillator strength of the C84 + transition obtained from the laboratory spectrum (f = 0.004). We find that 0.08% and 0.06% of the cosmic carbon abundance would be required to reproduce the 713.6 and 715.4 nm DIB, respectively. A ratio of cosmic C84 + to C60 + of ∼7% or ∼5% is derived based on the assignment of C84 + to the weakest DIBs at 713.6 or 715.4 nm, respectively. Both ratios are in good agreement with the C84 + to C60 + ratio measured in the laboratory under various experimental conditions.

POLYCYCLIC AROMATIC HYDROCARBONS AND THE DIFFUSE INTERSTELLAR BANDS: A SURVEY

We discuss the proposal of relating the origin of some of the diffuse interstellar absorption bands (DIBs) to neutral polycyclic aromatic hydrocarbons (PAHs) present in translucent interstellar clouds. An assessment of ionized PAHs will be examined in a future report. The spectra of several cold, isolated gas-phase PAHs have been measured in the laboratory under experimental conditions that mimic the interstellar conditions and are compared with an extensive set of astronomical spectra of reddened, early-type stars. This comparison provides—for the first time—accurate upper limits for the abundances of specific PAH molecules along specific lines of sight, something not attainable from IR observations alone. The comparison of these unique laboratory data with high-resolution, high signal-to-noise ratio spectra leads to two major findings: (1) a finding specific to the individual molecules that were probed in this study and, which leads to the clear and unambiguous conclusion that the abundance of these specific neutral PAHs must be very low in the individual translucent interstellar clouds that were probed in this survey (PAH features remain below the level of detection) and, (2) a general finding that neutral PAHs exhibit intrinsic band profiles that are similar to the profile of the narrow DIBs indicating that the carriers of the narrow DIBs must have close molecular structure and characteristics. This study is the first quantitative survey of neutral PAHs in the optical range and it opens the way for unambiguous quantitative searches of PAHs in a variety of interstellar and circumstellar environments.

Evidence for biodegradation products in the interstellar medium

AbstractThe interstellar absorption band centred on 2175 Å that is conventionally attributed to monodisperse graphite spheres of radii 0.02 μm is more plausibly explained as arising from biologically derived aromatic molecules. On the basis of panspermia models, interstellar dust includes a substantial fraction of biomaterial in various stages of degradation. We have modeled such an ensemble of degraded biomaterial with laboratory spectroscopy of algae, grass pigments, bituminous coal and anthracite. The average ulrtraviolet absorption profile for these materials is centred at 2175 Å with a full width at half maximum of 250 Å, in precise agreement with the interstellar extinction observations. Mid-infrared spectra also display general concordance with the unidentified interstellar absorption features found in a wide variety of astronomoical sources.

TD-DFT calculations of electronic spectra of hydrogenated protonated polycyclic aromatic hydrocarbon (PAH) molecules: implications for the origin of the diffuse interstellar bands?

We report the application of time-dependent density functional theory (TD-DFT) to the calculation of electronic spectra of hydrogenated protonated polycyclic aromatic hydrocarbon (PAH) molecules. The hydrogen atoms lie on the periphery of the PAH structure and those considered here may be written Hn-HPAH+, where n is even. It is found, in common with protonated PAH molecules, HPAH+, that some of the electronic transitions fall in the visible spectral region. The implications of the results are discussed in the context of the long-standing enigmatic astronomical problem of the diffuse interstellar absorption bands.

Relation between CH cation and neutral/molecular hydrogen

Observations of interstellar absorption bands of CH + molecule at 3957 and 4232 A were applied to determine column densities toward 53 stars. The targets were selected because the atomic and molecular hydrogen column densities are published. The data on CH + were acquired using four echelle spectrographs situated in both the Northern and Southern hemispheres. Spatial relations between column densities of CH + and those of molecular, atomic and total hydrogen show large scatter, suggesting there is no relation between abundances of methylidyne cation and hydrogen in any form.

Infrared Spectroscopy of Diamondoid Molecules: New Insights into the Presence of Nanodiamonds in the Interstellar Medium

Although they are relatively different in band shape, infrared features around 3.4-3.5 μm in the emission spectra of HD 97048 and Elias 1 and in the absorption spectra of various dense clouds have both been attributed to diamondoid molecules/particles. This assignment is based mainly on infrared spectra of hydrogenated diamond thin films and of diamond nanocrystals of known average size. Here we present an analysis of the astrophysical implications of recently reported solid-state 2.5-12.5 μm spectra of individual diamondoid molecules, up to the size of hexamantane (C26H30). These spectra provide the first experimental measurements of the infrared frequencies of this class of molecules. In addition, laboratory gas-phase infrared emission spectra of the three smallest members of the diamondoid family are reported, as well as theoretical spectra for some larger species. The present data set allows us to relate spectral signatures to the molecular size and structure. The spectra of tetrahedral diamondoids are found to be qualitatively different from those of lower symmetry species, which possibly explains the differences between the astrophysical emission and absorption spectra. Interestingly, the 3.53 μm band is clearly observed in the spectra of these small molecular diamondoids, whereas previous studies on nanodiamond particles found this band only for species larger than ≈50 nm. Our results support the assignment of the 3.43 and 3.53 μm emission features in HD 97048 and Elias 1 to diamondoids of a few nanometers in size as well as the suggestion that smaller diamondoid molecules contribute to the 3.47 μm interstellar absorption band.

Vibrational and Electronic Absorption Spectroscopy of Dibenzo[b,def]chrysene and Its Ions

The vibrational and electronic absorption spectra of dibenzo[b,def]chrysene (DBC) and its ions in argon matrixes have been recorded. Assignment of the observed infrared (IR) bands has been made by comparison with the density functional theory (DFT) computations of harmonic vibrational frequencies (with 6-31G(d,p) or 6-311+G(d,p) basis sets). Extensive time-dependent (TD) DFT calculations of vertical excitation energies have aided in the assignment of the experimental electronic absorption transitions. In general, the theoretical predictions are in good agreement with the observed ultraviolet and visible bands. By correlating IR and UV-visible band intensities (after UV photolysis), it has been shown that both DBC cations and anions are formed. The IR band intensity distributions of the DBC ions differ markedly from neutral DBC. A synthetic spectrum composed of neutral, cationic, and anionic DBC contributions compares reasonably well with the interstellar features of the "unidentified infrared" (UIR) bands from the reflection nebula NGC 7023. Finally, it is shown that the electronic absorption bands of the DBC ions lie in close proximity to several of the diffuse interstellar visible absorption bands (DIBs).

The Absence of Crystalline Silicates in the Diffuse Interstellar Medium

We have studied the dust along the line-of-sight towards the Galactic Center using Short Wavelength Spectrometer (SWS) data obtained with the Infrared Space Observatory (ISO). We focussed on the wavelength region from 8-13 micron which is dominated by the strong silicate absorption feature. Using the absorption profiles observed towards Galactic Center Sources (GCS) 3 and 4, which are C-rich Wolf-Rayet Stars, as reference objects, we are able to disentangle the interstellar silicate absorption and the silicate emission intrinsic to the source, toward Sgr A* and derive a very accurate profile for the intrinsic 9.7 micron band. The interstellar absorption band is smooth and featureless and is well reproduced using a mixture of 15.1% amorphous pyroxene and 84.9% of amorphous olivine by mass, all in spherical sub-micron-sized grains. There is no direct evidence for substructure due to interstellar crystalline silicates. We are able to determine an upper limit to the degree of crystallinity of silicates in the diffuse interstellar medium (ISM), and conclude that the crystalline fraction of the interstellar silicates is 0.2% (+/- 0.2%) by mass. This is much lower than the degree of crystallinity observed in silicates in the circumstellar environment of evolved stars, the main contributors of dust to the ISM. There are two possible explanations for this discrepancy. First, an amorphization process occurs in the ISM on a time scale significantly shorter than the destruction time scale, possibly caused by particle bombardment by heavyweight ions. Second, we consider the possibility that the crystalline silicates in stellar ejecta are diluted by an additional source of amorphous silicates, in particular supernovae.

The Organic Refractory Material in the Diffuse Interstellar Medium: Mid‐Infrared Spectroscopic Constraints

This is an analysis of the 4000¨1000 cm~1 (2.5¨10 km) region of the spectrum of diUuse interstellar medium (DISM) dust compared with the spectra of 13 materials produced in the laboratory which serve as analogs to the interstellar material. The organic signatures of extragalactic dust, carbonaceous chondritic material, and E. coli bacteria are also presented because these have been discussed in the literature as relevant to the diUuse interstellar medium. Spectral analysis of the DISM allows us to place signi—cant constraints on the applicability of proposed candidate materials. The spectra of candidate materials are evaluated using four spectral characteristics based on the interstellar data: (i) comparisons of the pro—le and subpeak positions of the 2940 cm~1 (3.4 km) aliphatic CH stretching-mode band, (ii) the ratio of the optical depth (O.D.) of the aliphatic CH stretch to the O.D. of the OH stretch near 3200 cm~1 (3.1 km), (iii) the ratio of the O.D. of the aliphatic CH stretch to the O.D. of the carbonyl band near 1700 cm~1 (5.9 km), and (iv) the ratio of the O.D. of the aliphatic CH stretch feature to the O.D. of the CH deformation modes near 1470 cm~1 (6.8 km) and 1370 cm~1 (7.25 km). We conclude that the organic refractory material in the diUuse interstellar medium is predominantly hydrocarbon in nature, possessing little nitrogen or oxygen, with the carbon distributed between the aromatic and aliphatic forms. Long alkane chains with n much greater than 4 or 5 are not major constituents H 3 Cw(CH 2 ) n w of this material. Comparisons to laboratory analogs indicate the DISM organic material resembles plasma processed pure hydrocarbon residues much more so than energetically processed ice residues. This result is consistent with a birth site for the carrier of the 3.4 km band in the out—ow region of evolved carbon stars. The organic material extracted from the Murchison carbonaceous meteorite and the spectrum of E. coli bacteria reveal spectral features in the 5¨10 km region that are absent in the DISM. Although the presence of unaltered circumstellar components in the Murchison meteorite has been established through several lines of evidence, it is unclear whether or not the aliphatic component which gives rise to the 3.4 km band is in that category. Considering the complete 2¨10 km wavelength region, there is no spectral evidence for a biological origin of the 3.4 km interstellar absorption band. The similarity of the aliphatic CH stretch region of dust from our own Galaxy compared with that of distant galaxies suggests that the organic component of the ISM is widespread and may be an important universal reservoir of prebiotic organic carbon. )

Hydrogenation of carbon grains by exposure to hydrogen atoms: Implications for the 3.4 μm interstellar absorption band

The 3.4 μm absorption band provides direct evidence for the presence of organic compounds in the diffuse interstellar medium. There is, however, a distinct absence of the band in the spectra of molecular cloud dust. The difference between diffuse and dense environments represents a strong constraint for any description of the formation and evolution of the band carrier material. Laboratory research under simulated interstellar medium conditions is of crucial interest for improving our understanding of the nature of this interstellar dust component. In fact, the composition of carbonaceous materials in the interstellar medium can vary in response to several evolutionary factors. Irradiation from UV photons and cosmic rays, as well as chemical interactions with the interstellar gas, can drive grain transformation. Here we discuss the results of experiments aimed at studying the interaction of nano-sized carbon grains with atomic hydrogen. H atoms induce the formation of CH bonds in carbon grains. After hydrogen exposure, the infrared spectrum of carbon grains is similar to that of the aliphatic component observed toward the Galactic center. The efficiency of the hydrogenation process and the amount of carbon which is necessary to reproduce the intensity of the interstellar feature are reported. Finally, implications for the formation and evolution of the organic materials responsible for the 3.4 μm band are discussed.