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

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  by Pam Graham  

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DIB Theories & Importance to Astrophysics

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The extensive list of proposed DIB carriers has at times included such diverse candidates as H-, H2, porphyrins, polycyclic aromatic hydrocarbons, nanodiamonds, fullerenes, nanotubes, and alien bacteria.

Merrill and Wilson (1938) [Merrill, P.W. and Wilson, O.C. (1938) Astrophys. J. 87, 9.] were the first to speculate, based on the dependence of DIB strength on IS reddening, that small solid particles were the carriers. However, the development of more refined techniques for measuring DIB strengths and reddenings has subsequently diminished, but not conclusively dismissed, support for dust grains as DIB carriers.5

Two arguments for keeping the dust grain possibility alive include: 1. Many families of DIBs corresponding to different carriers can be expected to emerge, and since only a few of the known structures have been thoroughly investigated, dust cannot be excluded as the origin of at least some of the remaining DIBs. 2. Different IS clouds may present a different ratio of large grains to small grains, altering the points in DIB strength-reddening diagrams that seem to exclude dust as carriers.6

closeup of dust
Porous chondrite interplanetary dust, an early candidate for the cause of Diffuse Interstellar Bands

One fascinating, but highly controversial line of questioning came from outside the astronomical community. In 1996, two laser spectroscopists from IBM, Peter Sorokin and James Glownia, suggested that the mystery compound is none other than the hydrogen molecule, H2, the simplest and most abundant molecule in space. Their model was able to account for the wavelengths of about 70 DIBs, but it also relied on a hydrogen molecule simultaneously absorbing a photon of visible light and a photon of ultraviolet light. Such two-photon hits are rare, and to get enough of them would require an extremely intense flux of ultraviolet light. Nevertheless, the search for DIB carriers had become so frustrating to scientists, Sorokin and Glownia's theory received enough interest to earn them some observing time on a telescope aboard the space shuttle Columbia in late 1996.

The H2 model of Sorokin and Glownia was eventually deemed an intriguing suggestion that overlooks several critical spectroscopic and astrophysical problems, and has ultimately been regarded as unproven and probably not viable.7

Why has all this effort, energy and funding been poured into the identification of such an evasive constituent of the distant, murky regions of our Universe?

In pursuing the origins of DIBs, we are tracing our chemical heritage, from the interstellar cloud that made the Solar System to the start of life on Earth. We need this knowledge as a basis to understand whether certain very complex chemical processes may occur in the ISM, especially on grain surfaces, and eventually to clarify whether life, also in its simplest form, is a ubiquitous phenomenon in the Universe.8

cosmic cloud
Molecular cloud of gas and dust, broken off from the Carina Nebula. Studying such clouds may hold clues to the origin of life on earth

Dense molecular clouds are seen to exist throughout our galaxy, and all planetary systems are believed to form from this material. So, the processes being studied in the search for DIB carriers are universal ones, i.e., whether the universe is in some sense hardwired to produce large quantities of prebiotic organic materials. The result would be the virtual assurance that when new planetary systems are formed, prebiotic organics would be present in the starting materials.9

Also, there remains the interesting possibility that some of these spectral features arise from new forms of matter or dust in the ISM and it is notable that new forms of carbon including fullerenes, nanotubes and graphene have only relatively recently become experimentally accessible. In fact, research attempts to simulate DIBs in the laboratory led to the accidental discovery in 1985 of the Buckminsterfullerene, or the "buckyball" carbon-60 molecule, for which the Nobel Prize was awarded in 1996.

Buckyballs possess unique chemical and physical properties that hold an array of possibilities for all the natural sciences. They are an entirely new material providing scientists with information about allotropes of carbon never before conceived. A few areas where buckyballs are proving valuable to research include drug treatments, medical diagnostics, nano scanning tunneling microscopy, electrical circuitry, lubricants, superconductors, and catalysts. The discovery of nanotubes in 1991 by S. Iijima has been by far the buckyball's most significant contribution to current research.10

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