Dust in the interstellar medium

The information on the nature and properties of the interstellar dust are presently obtained through the following observations  

The basic information on the nature of the dust has been obtained from analysis of interstellar extinction from the near-infrared to the far-ultraviolet spectral region, using ground-based telescopes and the first space-borne ultraviolet telescopes. The derived interstellar extinction curve is in most parts rather smooth and shows only one broad and strong absorption feature centered around 220 nm (cf. Fitzpatrick Massa 2007). This feature is explained by carbonaceous dust grains (Stecher Donn 1965) with a wide distribution of sizes. The true nature of the carbonaceous dust material remains still somewhat unclear, but seems to be some kind of amorphous carbon (cf. Draine 2003, 2004, for a detailed discussion). 

From theoretical considerations it was concluded in the 1960s that carbon (graphite) or silicate dust can condense in carbon-rich or oxygen-rich1 cool stars and that this dust can be driven out from the stars by radiation pressure (Hoyle  

Wickramasinghe 1962 Wickramasinghe etal. 1966). Condensation of some other solid phases was also proposed, e.g. of SiC in C-stars (Friedemann 1969), and iron and other solids in supernova (SN) ejecta as they cool by expansion (Hoyle Wickramasinghe 1970). 

Already the first infrared observations of late-type giant stars have revealed that many of them are indeed surrounded by thick dust shells (Woolf Ney 1969). These were rapidly found to consist of carbonaceous dust (some kind of soot) if the stellar spectrum indicates the star to be carbon-rich, and to be silicate dust (olivine, pyroxene) if the star is oxygen-rich (Gilman 1969). Since this dust is mixed into the interstellar medium due to mass loss by stellar winds, it was then assumed that silicate and carbon particles are abundant dust components in the interstellar medium. 

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A comprehensive book series which encompasses the complete coverage of carbon materials and carbon-rich molecules from elemental carbon dust in the interstellar medium to the most specialized industrial applications of elemental carbon and its derivatives. A great emphasis is placed on the most advanced and promising applications ranging from electronics to medicinal chemistry. The aim is to offer the reader a book series which not only consists of self-sufficient reference works, but one which stimulates further research and enthusiasm. 

Two types of models have been proposed that use this general picture as the basis for understanding volatile depletions in chondrites. Yin (2005) proposed that the volatile element depletions in the chondrites reflect the extent to which these elements were sited in refractory dust in the interstellar medium. Observations show that in the warm interstellar medium, the most refractory elements are almost entirely in the dust, while volatile elements are almost entirely in the gas phase. Moderately volatile elements are partitioned between the two phases. The pattern for the dust is similar to that observed in bulk chondrites. In the Sun s parent molecular cloud, the volatile and moderately volatile elements condensed onto the dust grains in ices. Within the solar system, the ices evaporated putting the volatile elements back into the gas phase, which was separated from the dust. Thus, in Yin s model, the chondrites inherited their compositions from the interstellar medium. A slightly different model proposes that the fractionated compositions were produced in the solar nebula by... 

The 10 p.m feature of chondritic IDPs has been compared with the 10 p.m feature of astronomical silicates. No particular IDP IR class consistently matches the —10 p.m feature of solar system comets or silicate dust in the interstellar medium (Sandford and Walker, 1985). However, the —10 p.m features of CP IDPs composed mostly of GEMS and submicrometer enstatite and forsterite crystals generally resemble those of comets and late-stage Herbig Ae/Be stars in support of the hypothesis that some CP IDPs are of cometary origin (Figure 10). 

The concentration of dust in the interstellar medium is far less than that of gas, about one particle per 1013 cubic centimeter for dust compared with one atom, molecule, or ion per cubic centimeter for gas. Comparatively little is known about the composition and origin... 

Approximately 99 % of the mass of the interstellar medium is in the form of gas (where denser regions are termed interstellar clouds) with the remainder primarily in the form of dust. The total mass of the gas and dust in the interstellar medium is about 15 % of the total mass of visible matter in the Milky Way. The exact nature and origin of interstellar dust grains is unknown, but they are presumably ejected from stars. One likely source is from red giant stars late in their lives. Interstellar dust grains are typically a fraction of a micron across, irregularly shaped, and composed of carbon an or silicates. 

In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

Table 5.2 shows that quite large molecules, of which the cyanopolyacetylenes form a remarkable group, have been detected. The presence of such sizeable molecules in the interstellar medium came as a considerable surprise. Previously, it was supposed that the ultraviolet radiation present throughout all galaxies would photodecompose most of the molecules, and particularly the larger ones. It seems likely that the dust particles play an important part not only in the formation of the molecules but also in preventing their decomposition. 

Molecular inventory An extension of the idea of molecular processing on dust grains in the interstellar medium to the surface of a comet... 

Dust Micron-sized particles of silicate in the interstellar medium responsible for short-wavelength scatter - dust particles become covered with water-ice mantels. 

Fullerene-Like Structures on Carbon Dust of the Interstellar Medium Their Role in Molecular Hydrogen Formation... 

Molecules interact with the surfaces of solids in almost every environment in the universe. In addition to purely intellectual interest, we customarily justify studying these interactions on technological grounds, heterogeneous catalysis and the fabrication of microchips being the most frequently listed applications. However the field is much more broadly relevant the adsorption and desorption of atoms and molecules on the surfaces of dust grains is very important to molecule formation in the interstellar medium, reactions on the surfaces of ice crystals is important in atmospheric chemistry and reactions at surfaces determine the behaviour of medical implants in our bodies. 

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. 

Laboratory studies of presolar dust grains also show that dust is formed in the mass ejected after an SN explosion, as will also be discussed in Section 2.2. Observations show that the ejected mass shells occasionally do form dust some time after the SN explosions (e.g. Bianchi Schneider 2007), but generally the efficiency of dust production seems to be rather low (Bianchi Schneider 2007 Zhukovska et al. 2008). Other important sources of stardust are red supergiants (mostly silicate dust). Most of the dust from red supergiants, however, is not expected to survive the shock wave from the subsequent SN explosion of the star (Zhukovska et al. 2008). Some dust is also formed by novae (Amari et al. 2001b), Wolf-Rayet stars (WRs, Crowther 2007), and luminous blue variables (LBVs, Voors et al. 2000), but the dust quantities formed by these are very small. Stardust - i.e. dust that is formed in stellar outflows or ejecta - in the interstellar medium is dominated by dust from AGB stars. 

The earliest detailed studies of silicate dust in protoplanetary disks targeted those brightest in the mid-infrared, where high quality spectra could be obtained even by severely flux-limited observations. Cohen Wittebom (1985) reported the earliest detection of crystalline silicate emission from the environment of young stars and interpreted it as evidence for dust having been transformed from its pristine state in the interstellar medium to the material known to be contained in the comets and perhaps primitive meteorites. Interestingly, this observation and explanation pre-dated the evidence that young stars are surrounded by disks and not by spherical envelopes. 

Jones A. P. (2000) Depletion patterns and dust evolution in the interstellar medium. J. Geophys. Res. 105, 10257—10268. 

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