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

 
  by Pam Graham  

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Articles

  1. Did molecules from space seed life in the cosmos?

    Dorminey, Bruce Astronomy 04-01-2008

    Sitting on Earth amid a sea of complex molecules, we can easily forget these building blocks of the physical world are much harder to come by outside our solar system. Large distances, low densities, and faint signals conspire to make molecule-hunting a difficult task. Still, dense molecular clouds like the one in which the solar system formed some 4.6 billion years ago showup throughout the galaxy.

    Astronomers discovered the first interstellar molecules in the 1930s. . . .

    Since the 1920s, astronomers have puzzled over the presence of what they now call Diffuse Interstellar Bands (DIBs). To date, researchers have found more than 500 DIBs in optical and ultraviolet spectra of stars throughout the Milky Way and a handful of nearby galaxies.

    The leading candidate for DIBs "carriers" are polycyclic aromatic hydrocarbons (PAHs). These molecules account for 20 percent of all cosmic carbon. On Earth, PAHs exist in everything from naphthalene, the active agent in mothballs, to the carcinogenic byproducts of petroleum refineries, soot, road tar, forest fires, and charbroiled meats.

    The view that DIBs could be related to some sort of PAH or other large, carbonchain molecule, such as fullerenes, has been around since the 1950s. But the idea is not without its critics. . . .

    For full-text documents see ProQuest's eLibrary Science

  2. Electronic spectroscopy of the nonlinear carbon chains

    Araki, Mitsunori; Cias, Pawel; Denisov, Alexey; Fulara, Jan; Maier, John P Canadian Journal of Chemistry 06-01-2004

    High-resolution spectroscopy of unsaturated carbon chain radicals is of interest in view of interstellar chemistry, as well as terrestrial discharge and flame processes. Electronic transitions of the poly acetylene cations HCnH+ (1-4) and radicals CnH (5) with even number carbons have now been studied. Recently a nonlinear planar carbon chain radical C6H4+, H-C≡C-C≡C-CH=CH2+, was detected in a supersonic planar discharge with cavity ring down spectroscopy in the 604 nm region (6). Therefore, the related C4H4+ and C8H4+ nonlinear chains were expected to be produced in the plasma discharge in a sufficient concentration for detection.

    Hydrogenated nonlinear chains can exist in dark interstellar clouds as well as linear ones, e.g., the column densities of C2O, H2CCO, and CH3CHO are of the same order of magnitude in Taurus Molecular Cloud-1 (7). However, the intensity of rotational transitions for a linear molecule with a smaller rotational partition function is stronger than for an asymmetric top, and thus, a long nonlinear carbon chain has not yet been detected by radio astronomy, even though the population may be comparable. In the case of electronic transitions, an asymmetric top molecule is not at a disadvantage compared with the linear one. Thus, the nonlinear chains can be considered as candidates, causing the diffuse interstellar band (DIB) absorptions in much the same way as the linear series (8). . . .

    For full-text documents see ProQuest's eLibrary Science

  3. Explore the southern Milky Way's dark clouds

    Crossen, Craig Astronomy 09-01-2008

    Dusty webs sprawling across the galaxy's richest star fields make for must-see observing. Our Milky Way Galaxy is a dirty place. So much soot and dust occupies interstellar space that we see less than 20 percent of the light from the brilliant Perseus Double Cluster (NGC 869 and NGC 884). But for all dust takes away, it also helps create some of the finest cosmic vistas. In fact, we usually take note of them only when they lie between us and a star-rich patch of the galaxy.

    Microscopic grains of carbon (ordinary soot), ice (water), silicates (sand), and even some metals (notably iron from certain types of supernovae) compose interstellar dust. The dust gathers in clumps and clouds. The thickest clouds shield their interiors from starlight, which makes them cool enough that spaces ever-present hydrogen gas can exist in its molecular form. That's why astronomers call the densest of these regions molecular clouds.

    For full-text documents see ProQuest's eLibrary

Historical Newspapers

  1. Luminiferous Ether

    Chicago Daily Tribune. Chicago, Ill.: Jul 6, 1879. pg. 13

    Abstract (Summary) A writer in last Sunday's TRIBUNE disputes the existence of the resisting medium of ether, and affirms that space is a vacuum.

    Original Newspaper Image (PDF)

  2. A question of Stardust

    N G Matthew. London (UK): Jan 31, 1972. pg. 8

    Original Newspaper Image (PDF)

  3. Laser Physicist Gets No Respect From Astronomers

    The Christian Science Monitor. Nov 19, 1996. p. 12

    Abstract (Summary) PETER SOROKIN is a world-class laser physicist. But when it comes to astronomy, he admits he's a rank amateur. That has given him a unique perspective on a 75-year-old astronomical mystery--along with a bruising intellectual experience.

    Original Newspaper Image (PDF)

Taken from ProQuest's Historical Newspapers.

Dissertations

  1. Diffuse interstellar bands in damped Lyman-alpha and starburst galaxies

    by Lawton, Brandon, Ph.D., New Mexico State University, 2008 , 211 pages

    Abstract (Summary)
    I present a study of diffuse interstellar bands (DIBs) in seven moderate redshift HI-selected damped Lyα galaxies and 18 low to moderate redshift NaI-selected starbursts. I have developed a spectral analysis method for finding and calculating limits for the weak DIB absorption profiles. The spectral analysis is optimized for both resolved and unresolved features and also assesses the accuracy of equivalent width limits by calculating the effects caused by errors in the DIB intrinsic rest wavelength, DIB rest FWHM, instrumental resolution, and continuum placement. This spectral analysis technique is used to calculate robust equivalent width limit in the HI-selected DLA sample. The DIBs in DLAs are weaker than expected, based on Galactic observations and the large abundance of H I. The DIB limits put constraints on the DLA reddening and gas-to-dust ratios. Four of the seven DLAs have sufficiently good limits to constrain DLA reddenings to less than 0.08. I constrain the DLA gas-to-dust ratios as being comparable to the Magellanic Clouds for the four DLAs with the best limits. I detect DIBs in 14 of the 18 NaI-selected starbursts. The λ5780 DIB is especially prevalent in the starburst sample. I perform Kendall ? correlation tests on the DIB equivalent widths and velocities with the NaI outflow properties and the intrinsic starburst properties. The λ5780 DIB is weakly to moderately correlated with the column densities and velocities of the NaI outflow gas, but not with galaxy properties. The λ5780 DIB velocity is moderately correlated with the NaI outflow velocity, but it is also moderately correlated with velocity separation from the NaI outflow gas. As the NaI outflow velocity increases, so does the velocity separation between the DIB absorbing gas and the NaI outflow gas. The λ5780 DIB shows a weak anti-correlation with the column density of NaI outflow gas, which is opposite of what is observed in the Galaxy. I argue that the correlation tests indicate that the λ5780 DIB absorbing gas is influenced by the NaI gas abundance and kinematics, but the DIB absorbing gas is not entrained in the largest N (NaI) outflow gas.

    For full-text documents see ProQuest's Dissertations & Theses Database

  2. Matrix-isolation spectroscopy of triplet mono- and dialkyl-substituted pentadiynylidenes and interpretation of their electronic absorption spectra

    by Thomas, Phillip Sterling, Ph.D., The University of Wisconsin - Madison, 2007 , 417 pages

    Abstract (Summary)
    Spectroscopic characterization of triplet carbenes of the R-C 5 -R' structural type is the primary focus of this work. These species are highly reactive; however, they are stable in cryogenic matrices at temperatures on the order of 10 K. Under these conditions spectroscopic analysis can be undertaken. Following previous work on the parent compound, HC 5 H, triplet MeC5H is investigated (Chapter 1), as are analogs t -BuC 5 H and MeC 5 Me (Chapter 2). All three of the substituted C5 molecules discussed exhibit pronounced vibrational structure in their optical spectra; thus, an analysis of their spectral features is a matter of interest. These species are similar structurally to a number of identified interstellar molecules, providing hopes that the electronic spectra of C5 compounds may provide clues to the nature of the "Diffuse Interstellar bands", a series of yet-unassigned optical absorption bands in space. Chapter 3 outlines the interpretation of the electronic absorption spectra of HC5H and substituted analogs with the help of experimental data, computations, and Franck-Condon simulations. The methodology behind the simulations is presented in Chapter 4.

    Finally, Chapter 5 focuses on the computational treatment of the potential energy surface of propadienylidene (H2 C=C=C:), a known interstellar molecule whose photochemical rearrangement mechanisms have yet to be explained computationally.

    For full-text documents see ProQuest's Dissertations & Theses Database

  3. Line profiles in the diffuse interstellar bands

    by Drosback, Meredith Marie, Ph.D., University of Colorado at Boulder, 2006 , 184 pages

    Abstract (Summary)
    For eighty-five years, the carriers of the Diffuse Interstellar Bands (DIBs) have eluded identification. From the first studies aimed at simply determining whether these absorption features were truly interstellar to the most recent high resolution spectroscopic studies of the detailed substructure in some of the DIBs, the ultimate goal has remained the same: to identify a carrier of the DIBs. It seems that nearly as many carriers have been proposed, and ultimately rejected, as there are DIBs. The purpose of this thesis is to present several observational studies of characteristics of the DIBs in an effort to elucidate some information regarding the behavior of the carriers, and perhaps highlight some possible carrier types.

    Studies of the line profiles of the DIBs are useful ways of characterizing the type of carrier that could be producing a particular DIB. The profiles of several DIBs, including those at 4428 and 6284 Å, are discussed here. These DIB profiles are measured without the difficult modeling and removal of the underlying stellar features, yet we are still able to determine some physical features of these DIBs. The 4428 and 6284 Å DIBs in particular appear to be well fit by a Lorentzian profile. The Lorentzian profile implies a gas-phase molecular carrier for these DIBs and the width of the profile reflects a very short lifetime for the excited state in the transition. The 5487 and 6177 Å profiles appear in some cases to be best described by a Lorentzian profile, but other spectra appear to be best fit with a Gaussian. The results of these two DIB profiles are ultimately inconclusive. The narrow 6140 Å DIB exhibits a different profile from its broader counterparts, and is best fit by a Gaussian in this study.

    Correlation studies of the DIBs are performed in an effort to understand more about the behavior of the DIB carriers with respect to either their environment, or to other DIB carriers. If the strengths of two (or more) DIBs are well correlated, then that is an indication that their carriers are either closely related or, in the case of a perfect correlation, the same. The tight correlation between the two DIBs at 6196 and 6614 Å indicates that it is likely that these two DIBs result from transitions within a single molecule.

    A study of the DIBs in a specific line of sight can provide useful information regarding the DIBs as well as characteristics of the local interstellar environment. BQ Cam is a highly reddened sightline (E B-V = 1.87) due to a long path length to the background star (7 kpc). The DIBs appear as strong features in the spectrum of this sightline. The strengths of several DIBs (as measured by their equivalent widths) are shown to be consistent with the well established relationship to interstellar reddening. Although they appear to be quite strong in the spectrum, they are not abnormally strong and simply extend the relationship to a more highly reddened sightline than average.

    For full-text documents see ProQuest's Dissertations & Theses Database