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Diffuse Interstellar Bands: A Cosmic Mystery
(Released November 2009)

 
  by Pam Graham  

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  1. Detailed maps of interstellar clouds in front of omega Centauri: small-scale structures in the Galactic disc-halo interface

    Jacco Th Loon, Keith T. Smith, Iain McDonald, Peter J. Sarre, Stephen J. Fossey and Robert G. Sharp.

    Monthly Notices of the Royal Astronomical Society, Vol. 399, No. 1, 11 Oct 2009, pp. 195-208.

    The multiphase interstellar medium (ISM) is highly structured, on scales from the size of the Solar System to that of a galaxy. In particular, small-scale structures are difficult to study and hence are poorly understood. We used the multiplex capabilities of the AAOmega spectrograph at the Anglo-Australian Telescope to create a half-square-degree map of the neutral and low-ionized ISM in front of the nearby ( similar to 5 kpc), most massive Galactic globular cluster, omega Centauri. Its redshifted, metal-poor and hot horizontal branch stars probe the medium-strong Ca ii K and Na i D2 line absorption, and weak absorption in the lambda 5780 and lambda 5797 diffuse interstellar bands (DIBs), on scales around a parsec. The kinematical and thermodynamical picture emerging from these data is that we predominantly probe the warm neutral medium and weakly ionized medium of the Galactic Disc-Halo interface, similar to 0.3-1 kpc above the mid-plane. A comparison with Spitzer Space Telescope 24 mu m and DIRBE-IRAS maps of the warm and cold dust emission confirms that both Na i and Ca ii trace the overall column density of the warm neutral and weakly ionized medium. Clear signatures are seen of the depletion of calcium atoms from the gas phase into dust grains. Curiously, the coarse DIRBE-IRAS map is a more reliable representation of the relative reddening between sightlines than the Na i and Ca ii absorption-line measurements, most likely because the latter are sensitive to fluctuations in the local ionization conditions. The behaviour of the DIBs is consistent with the lambda 5780 band being stronger than the lambda 5797 band in regions where the ultraviolet radiation level is relatively high, as in the Disc-Halo interface. This region corresponds to a sigma -type cloud in which Ca i and small diatomic molecules such as CH and CN are usually absent. In all, our maps and simple analytical model calculations show in unprecedented detail that small-scale density and-or ionization structures exist in the extra-planar gas of a spiral galaxy.

  2. Diffuse interstellar absorption bands

    FY Xiang, SL Liang and AG Li.

    Science in China Series G, Vol. 52, No. 4, Apr 2009, pp. 489-501.

    The diffuse interstellar bands (DIBs) are a large number of absorption bands that are superposed on the interstellar extinction curve and are of interstellar origin. Since the discovery of the first two DIBs in the 1920s, the exact nature of DIBs still remains unclear. This article reviews the history of the detections of DIBs in the Milky Way and external galaxies, the major observational characteristics of DIBs, the correlations or anti-correlations among DIBs or between DIBs and other interstellar features (e.g. the prominent 2175 Angstrom extinction bump and the far-ultraviolet extinction rise), and the proposed candidate carriers. Whether they are also present in circumstellar environments is also discussed.

  3. Diffuse interstellar absorption bands

    FY Xiang, SL Liang and AG Li.

    Science in China Series G, Vol. 52, No. 4, Apr 2009, pp. 489-501.

    The diffuse interstellar bands (DIBs) are a large number of absorption bands that are superposed on the interstellar extinction curve and are of interstellar origin. Since the discovery of the first two DIBs in the 1920s, the exact nature of DIBs still remains unclear. This article reviews the history of the detections of DIBs in the Milky Way and external galaxies, the major observational characteristics of DIBs, the correlations or anti-correlations among DIBs or between DIBs and other interstellar features (e.g. the prominent 2175 Angstrom extinction bump and the far-ultraviolet extinction rise), and the proposed candidate carriers. Whether they are also present in circumstellar environments is also discussed.

  4. NGC 2770: A SUPERNOVA Ib FACTORY?*

    Christina C. Thone, Michal J. Michalowski, Giorgos Leloudas, et al.

    Astrophysical Journal, Vol. 698, No. 2, Jun 2009, pp. 1307-1320.

    NGC 2770 has been the host of three supernovae (SNe) of Type Ib during the last ten years, SN 1999eh, SN 2007uy, and SN 2008D. SN 2008D attracted special attention due to the serendipitous discovery of an associated X-ray transient. In this paper, we study the properties of NGC 2770 and specifically the three SN sites to investigate whether this galaxy is in any way peculiar to cause a high frequency of SNe Ib. We model the global spectral energy distribution of the galaxy from broadband data and derive a star formation and SN rate comparable to the values of the Milky Way. We further study the galaxy using longslit spectroscopy covering the major axis and the three SN sites. From the spectroscopic study we find subsolar metallicities for the SN sites, a high extinction and a moderate star formation rate. In a high-resolution spectrum, we also detect diffuse interstellar bands in the line of sight toward SN 2008D. A comparison of NGC 2770 to the global properties of a galaxy sample with high SN occurrence (>= 3 SN in the last 100 years) suggests that NGC 2770 is not particularly destined to produce such an enhancement of observed SNe. Its properties are also very different from gamma-ray burst host galaxies. Statistical considerations on SN Ib detection rates give a probability of ~1.5% to find a galaxy with three Ib SNe detected in ten years. The high number of rare Ib SNe in this galaxy is therefore likely to be a coincidence rather than special properties of the galaxy itself. NGC 2770 has a small irregular companion, NGC 2770B, which is highly star-forming, has a very low mass and one of the lowest metallicities detected in the nearby universe as derived from longslit spectroscopy. In the most metal poor part, we even detect Wolf-Rayet (WR) features, which is at odds with most current models of WR stars which require high metallicities.

  5. Reaction dynamics of phenyl radicals in extreme environments: a crossed molecular beam study

    Xibin Gu and Ralf I. Kaiser.

    Accounts of Chemical Research, Vol. 42, No. 2, Feb 17 2009, pp. 290-302.

    Polycyclic aromatic hydrocarbons (PAHs)organic compounds that consist of fused benzene ringsand their hydrogen-deficient precursors have attracted extensive interest from combustion scientists, organic chemists, astronomers, and planetary scientists. On Earth, PAHs are toxic combustion products and a source of air pollution. In the interstellar medium, research suggests that PAHs play a role in unidentified infrared emission bands, diffuse interstellar bands, and the synthesis of precursor molecules to life. To build clean combustion devices and to understand the astrochemical evolution of the interstellar medium, it will be critical to understand the elementary reaction mechanisms under single collision conditions by which these molecules form in the gas phase. Until recently, this work had been hampered by the difficulty in preparing a large concentration of phenyl radicals, but the phenyl radical represents one of the most important radical species to trigger PAH formation in high-temperature environments. However, we have developed a method for producing these radical species and have undertaken a systematic experimental investigation. In this Account, we report on the chemical dynamics of the phenyl radical (C(6)H(5)) reactions with the unsaturated hydrocarbons acetylene (C(2)H(2)), ethylene (C(2)H(4)), methylacetylene (CH(3)CCH), allene (H(2)CCCH(2)), propylene (CH(3)CHCH(2)), and benzene (C(6)H(6)) utilizing the crossed molecular beams approach. For nonsymmetric reactants such as methylacetylene and propylene, steric effects and the larger cones of acceptance drive the addition of the phenyl radical to the nonsubstituted carbon atom of the hydrocarbon reactant. Reaction intermediates decomposed via atomic hydrogen loss pathways. In the phenyl-propylene system, the longer lifetime of the reaction intermediate yielded a more efficient energy randomization compared with the phenyl-methylacetylene system. Therefore, two reaction channels were open: hydrogen losses from the vinyl and from the methyl groups. All fragmentation pathways involved tight exit transition states. In the range of collision energies investigated, the reactions are dictated by phenyl radical addition-hydrogen atom elimination pathways. We did not observe ring closure processes with the benzene ring. Our investigations present an important step toward a systematic investigation of phenyl radical reactions under single collision conditions similar to those found in combustion flames and in high-temperature interstellar environments. Future experiments at lower collision energies may enhance the lifetimes of the reaction intermediates, which could open up competing ring closure channels to form bicyclic reaction products.

  6. Sidelight on the Diffuse Interstellar Bands Problem

    Frederic Zagury.

    Open Astronomy Journal, Vol. 2, No. , 2009, pp. 58-62.

    I investigate the conditions upon which atmospheric absorption may participate to the observation of a diffuse interstellar band (DIB), and the implications it would have. A necessary condition is that the spectrum of reddened stars comprises a few percent of starlight forward scattered by the interstellar cloud on the line of sight. Reciprocally, this scattered starlight could, in part, explain the complexity of the DIB spectrum and several observed DIB properties. It will also affect the interstellar extinction curve and the value of the RV parameter.

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

    Mark Hammonds, Amit Pathak and Peter J. Sarre.

    Physical chemistry chemical physics : PCCP, Vol. 11, No. 22, Jun 14 2009, pp. 4458-4464.

    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.

  8. Towards a comprehensive electronic database of polycyclic aromatic hydrocarbons and its application in constraining the identities of possible carriers of the diffuse interstellar bands

    Xiaofeng Tan.

    Spectrochimica acta.Part A, Molecular and biomolecular spectroscopy, Vol. 71, No. 5, Jan 2009, pp. 2005-2011.

    A theoretical approach is developed to pre-select individual polycyclic aromatic hydrocarbons (PAHs) as possible carriers of the diffuse interstellar bands (DIBs). In this approach, a computer program is used to enumerate all PAH molecules with up to a specific number of fused benzene rings. Fast quantum chemical calculations are then employed to calculate the electronic transition energies, oscillator strengths, and rotational constants of these molecules. An electronic database of all PAHs with up to any specific number of benzene rings can be constructed this way. Comparison of the electronic transition energies, oscillator strengths, and rotational band contours of all PAHs in the database with astronomical spectra allows one to constrain the identities of individual PAHs as possible carriers of some of the intense narrow DIBs. Using the current database containing up to 10 benzene rings we have pre-selected 8 closed-shell PAHs as possible carriers of the famous lambda6614 DIB.

  9. Detection of the 2175 Aa extinction feature and 21-cm absorption in two Mgii systems at z similar to 1.3

    R. Srianand, N. Gupta, P. Petitjean, P. Noterdaeme and DJ Saikia.

    Monthly Notices of the Royal Astronomical Society: Letters, Vol. 391, No. 1, 21 Nov 2008, pp. L69-L73.

    We have discovered two dusty intervening Mgii absorption systems at z similar to 1.3 in the Sloan Digital Sky Survey (SDSS) data base. The overall spectra of both quasi-stellar objects (QSOs) are red (u-K > 4.5mag) and are well modelled by the composite QSO spectrum reddened by the extinction curve from the Large Magellanic Cloud (LMC2) Supershell redshifted to the rest frame of the Mgii systems. In particular, we detect clearly the presence of the ultraviolet (UV) extinction bump at lambda sub(rest) similar to 2175Aa. Absorption lines of weak transitions like Siii lambda 1808, Crii lambda 2056, Crii+ Znii lambda 2062, Mnii lambda 2594, Caii lambda 3934 and Tiii lambda 1910 from these systems are detected even in the low signal-to-noise ratio and low-resolution SDSS spectra, suggesting high column densities of these species. The depletion pattern inferred from these absorption lines is consistent with that seen in the cold neutral medium of the LMC. Using the LMC A sub(V) versus N(Hi) relationship, we derive N(Hi) similar to 6 10 super(21)cm super(-2) in both systems. Metallicities are close to solar. Giant Metrewave Radio Telescope (GMRT) observations of these two relatively weak radio QSOs (f sub( nu ) similar to 50 mJy) resulted in the detection of 21-cm absorption in both the cases. We show that the spin temperature of the gas is of the order of or smaller than 500 K. These systems provide a unique opportunity to search for molecules and diffuse interstellar bands at z > 1.

  10. Diffuse interstellar bands in M 33

    MA Cordiner, KT Smith, NLJ Cox, et al.

    Astronomy & Astrophysics, Vol. 492, No. 1, Dec 2008, pp. L5-L8.

    Aims. We investigate the diffuse interstellar band (DIB) spectrum in the interstellar medium of the Local Group spiral galaxy M 33. Methods. Optical spectra of the M 33 supergiant star J013346.96+303642.8 were taken at a resolving power of similar to 3000 using the DEIMOS spectrograph of the W. M. Keck Observatory. Results. We report the first detection and measurement of DIBs in M 33. The lambda\lambda5705, 5780, 5797, 6203, 6269, 6283, and 6613 DIBs are detected at velocities matching the M 33 ion{i} absorption lines. The lambda4428 DIB is tentatively detected. The stellar spectrum is classified as A3 Ia and the line-of-sight interstellar reddening is derived. Diffuse interstellar band (DIB) equivalent widths and radial velocities are presented. Conclusions. The overall spectrum of DIBs observed in M 33 is found to be similar to that observed in the Milky Way. However, along this line-of-sight, the M 33 DIB equivalent widths per unit reddening are large compared with those typically observed in the Galaxy.

  11. Protonated PAHs as carriers of diffuse interstellar bands

    A. Pathak and PJ Sarre.

    Monthly Notices of the Royal Astronomical Society: Letters, Vol. 391, No. 1, 21 Nov 2008, pp. L10-L14.

    We report the first results of Time-dependent Density Functional Theory calculations of electronic transitions of protonated stable neutral polycyclic aromatic hydrocarbon (PAH) molecules. Unlike most neutral closed-shell PAHs which generally absorb strongly in the ultraviolet (UV) or near-UV, the corresponding isoelectronic protonated forms of PAH molecules such as coronene and ovalene are predicted to have absorptions in the visible spectral region. Combining this with other astrophysical, chemical and physical aspects, it is concluded that the protonated forms of PAHs represent a strong class of candidate carriers of diffuse interstellar bands.