Interstellar dust infrared

A Dirty Silicate Story A friend of one of the authors is an astronomer—as well as a professional mineral dealer—who became interested in dirty silicates as candidates for interstellar dust. He therefore selected for determination of the blackest natural silicate mineral in his possession, the coal-black mineral hornblende, which contains a high concentration of impurities such as iron. A slice about 100 jam thick was polished, and transmission was measured in a recording spectrophotometer. The fact that appreciable light was transmitted for all near-infrared and visible wavelengths indicated that k was rather small. Calculations indeed confirmed that k was less than 10 4 between about 6 and 0.3 jum. And yet this was the blackest silicate in the possession of a professional collector. It is not easy to find A = 0.01 in the band gap region of... 

Again, the emissivity varies as 1/A2 in the far infrared. But various observations of emission from interstellar dust suggest that the wavelength dependence of emissivity is closer to 1/A than to 1/A2 (Seki and Yamamoto, 1980). This may be a consequence of the failure of the conditions underlying the 1/A2 dependence—the particles are crystalline and spherical—to be satisfied. 

From these considerations we conclude that the failure of the emission spectrum of interstellar dust to vary as 1/A2 in the far infrared, which is predicted for small crystalline spheres, may be the result of either noncrystallinity or nonsphericity (or both). Therefore, the infrared emission spectrum may not prove to be as uniquely diagnostic of interstellar grain characteristics as it once was thought to be. 

Kratschmer, W., and D. R. Huffman, 1979. Infrared extinction of heavy ion irradiated and amorphous olivine, with applications to interstellar dust, Astrophys. Space Sci., 61, 195-203. 

The term unidentified infrared emission is used to refer to the long-known emission features of interstellar dusts in the spectral region from just over 3,000 cm-1 to below 800 cm-1 (Gillett et al. 1973). These features comprise sharp IR bands at 2,920,1,610, and 880 cm-1, as well as a broader envelope near 1,300 cm-1. In addition, a recurrent mode at 3,050 cm-1, a weak mode near 1,450 cm-1, and a shoulder near 1,150 cm-1 are observed. These spectral features can all be attributed to vibrational modes of hydrogenated carbon species, as summarized in Table 2.1. The chemical structure of these species remains the subject of debate. Furthermore, a number of carbon-rich astronomical objects reveal an emission feature in the far-IR at 490 cm-1, of unclear attribution (Kwok et al. 1989). 

The discovery of C60 by Kroto and coworkers (1985) was motivated in part by the interstellar dust problem. C60 would seem to be an ideal candidate, as it is spherical and graphite-like, it forms spontaneously in harsh environments with carbon dust, and is stable in intense radiation fields, a condition analogous to that found in the diffuse ISM (Kroto and Jura 1992). In fact, the observation of two DIBs at 957.7 and 963.2 nm are tentatively considered the first evidence of C60+ in interstellar dust (Foing and Ehrenfreund 1997). Moreover, a mixture of hydrides of C60 is shown to exhibit spectral features remarkably similar to those seen in the unidentified infrared emission (Stoldt et al. 2001). The UV absorption spectrum of synthetic C60H36 was also observed to possess abroad bump at 217.5 nm (Cataldo 2003). 

Greenberg, J. M. Proceedings of Workshop on Laboratory and Observational Infrared Spectra of Interstellar Dust, Wolstencraft and Greenberg, eds.. Occasional Reports of the Royal Observatory, Edinburgh 12,82 (1984b)... 

Infrared absorption spectroscopy of interstellar clouds shows that the interstellar dust population varies with the line of sight, yet it maintains a similar character. In particular, submicron-sized amorphous silicate grains are the dominant component in every direction. The absence of crystalline grains is likely the result of rapid amorphization by the interstellar radiation field. 

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. 

What is in the nucleus of the Milky Way If we look with optical telescopes, we see nothing. The interstellar dust obscures the optical light. The center of the Milky Way does, however contain very strong sources of radio waves, infrared light, and x rays. One such source, called Sagittarius A", appears to lie at the precise center of the galaxy, the point about which the entire system rotates. 

The polycyclic aromatic hydrocarbons (PAHs) have been inferred to exist in the interstellar dust by the correlation of their general infrared spectral characteristics with observed celestial infrared emission bands [13-15]. 

Keywords infrared - galaxy classification - interstellar dust - star formation - interacting... 

See isowww.estec.esa.nl/science/formany infrared spectra of the interstellarmedium. In many of these, features of the interstellar dust are clearly shown. 

Plate 2 illustrates the dramatic difference between the visible and infrared image of the sky looking towards Orion. The infrared radiation that dominates the IR image of the constellation (colored red in the figure) comes from the blackbody radiation of the interstellar dust particles comprising what is known as... 

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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