Interstellar dust composition

Electromagnetic stellar radiation which has passed through regions containing interstellar dust particles. The values of the absorbed or scattered wavelengths make it possible to draw conclusions on the chemical composition or the physical properties of the particles. 

In 2006 Beauchamp reported on the expanding role of mass spectrometry in developing new instruments for laboratory simulations and in situ exploration of the space environment.13 The cometary and interstellar dust analyzer ( ), which was designed for the direct analysis of the space environment and mounted on the spacecraft, is a time-of-flight mass spectrometer for measuring, in situ, the chemical composition and the original mass of the individual dust grains. 

Understanding the mechanism for H2 formation from H atoms in space is an old and important problem in astrophysics. It is generally accepted that H atom recombination takes place on interstellar dust particles [105]. The exact composition of these dust particles is not known, but there is spectroscopic evidence that they contain graphitic components, and this has stimulated several electronic structure studies of the H-graphite interaction [86, 89, 106-109]. The recent DFT studies of Sidis and co-workers [108, 109] and Sha and Jackson [89], who modeled graphite as a coronene molecule and a slab, respectively, are in general agreement. We review the results of these studies in Section 3.1. 

Cameron (1973) speculated that grains from stellar sources survive in the interstellar medium, become incorporated into bodies of the Solar System, and may be found in meteorites, because some meteorites represent nearly unprocessed material from the time of Solar System formation. These grains may be identified by unusual isotopic abundance ratios of some elements, since material from nuclear burning zones is mixed at the end of the life of stars into the matter from which dust is formed. Indeed, these presolar dust grains3 were found in the late 1980s in meteorites (and later also in other types of primitive Solar System matter) and they contain rich information on their formation conditions and on nucleosynthetic processes in stars (see Section 2.2). By identifying such grains in primitive Solar System matter it is possible to study the nature and composition of at least some components of the interstellar dust mixture in the laboratory. 

From infrared spectroscopy it is very difficult to obtain the composition of the amorphous silicates. This is because the spectral signature observed is a combination of grain composition, shape, size, and structure, making it difficult to isolate the pure amorphous silicate signal. This, in combination with the relatively small spectral changes caused by the composition of the silicates, makes it hard to get a definitive answer in most cases. In the case of interstellar dust we have a unique opportunity the grains are very small and (almost) all silicates are amorphous. 

In order to investigate the primary processes responsible for photocatalytic reactions Moesta and Breuer (1968) performed experiments in which simple gases, some examples of which are given later, were adsorbed on clean metal surfaces and subsequently irradiated by UV light. The different metals used in these experiments certainly do not resemble the chemical and physical composition of interstellar dust grains, but they at least offer well defined surfaces and thus avoid introducing errors due to impurities which may lead to a misunderstanding of the primary processes. 

The interstellar carbon grains contain both aliphatic and aromatic C—H and C—C bonds both are observed in absorption and emission in the ISM, but beyond this their exact composition is not known. Given the seemingly uniform elemental abundances in our local ISM we might expect that the dust composition would also be chemically uniform. However, the inferred elemental composition of silicates in the ISM... 

Jessberger E. K., Kissel J., Fechtig H., and Krueger F. R. (1989a) On the average chemical composition of cometary dust. In Evolution of Interstellar Dust and Related Topics, New Holland, New York, 455p. 

The second of the two tails, the dust tail, consists of tiny particles of matter expelled from the coma and nucleus by the Sun s radiative pressure. This tail is visible only because the particles of which it is composed reflect sunlight. The composition of cometary dust is thought to be similar to that of interstellar dust, but much remains to be learned about this portion of the comet s structure. In April 2003, NASA launched an experiment to obtain better data about the composition and structure of cometary dust. High-altitude research airplanes were flown through the upper atmosphere toward the tail of Comet Grigg-Skjellerup. Special collectors in the planes scooped up samples of the cometary dust. Analysis may take years. 

Over the past thirty years tremendous strides have been made in our understanding of the complex chemistry in dense, dark, interstellar molecular clouds. This has come about because of fundamental advances in observational infrared astronomy and laboratory astrophysics. Thirty years ago the composition of interstellar dust was largely guessed at the concept of ices in... 

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