Interstellar dust environments

Several different types of this dust are distinguished by astronomers. On average, interstellar dust resides in widely separated diffuse clouds. But there are also dense regions of gas and dust into which little ultraviolet radiation can penetrate, thereby providing an environment for the formation of complex molecules these are referred to as molecular clouds. Clouds of particles expelled by cooler stars into the regions around them are called circumstellar... 

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. 

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). 

Although man s environment, from the interstellar dust to the earth beneath his feet, is composed of finely divided material his knowledge of the properties of such material is surprisingly slight. For many years scientists have accepted that matter can exist as solids, liquids or gases Although the dividing line between the states may often be rather blurred this classification has been upset by powders which at rest are solids, when aerated may behave as liquids and when suspended in a gas take on some of the properties of the gas. 

Chondrites, the most primitive of all meteorites, formed in dynamic energetic, dust-rich zones in the solar nebula. In this environment, dust/gas ratios were constantly changing, temperatures fluctuated through 1,000 K, with multiple cycles of melting, evaporation, condensation, and aggregation. In addition there were influxes of matter from the interstellar dust and the periodic removal of batches of chondritic material to small planetesimals. In this section we explore how the most primitive materials of the solar system were formed and what they can tell us about processes during the condensation of the solar nebula. These materials include chondrules, refractory inclusions (CAIs), and amoeboid olivine aggregates (AOAs), the oldest component parts of chondritic meteorites. 

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. 

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. 

Waelkens et al., 1996) massive pre-main sequence stars surrounded with disks of dust and gas. Herbig Ae/Be stars even show transient gas features in their spectra that have been interpreted as comets falling into the star (Beust et al., 1994). The presence of the olivine feature in comets and circumstellar disk systems and the lack of it in interstellar and molecular clouds, the parental materials for star and planetary formation, is somewhat of a conundrum. A common astronomical interpretation is that interstellar grains are amorphous silicates and when warmed in a circumstellar disk environment, they anneal to produce crystalline materials. The other possibility is that olivine in comets and disks condenses from vapor produced by evaporation of original interstellar materials. 

Second, it is possible to determine the abundances of a wide variety of elements in DLAs with higher precision than in most other astrophysical environments in the distant universe. In particular, echelle spectra obtained with large telescopes can yield abundance measures accurate to 10-20% (e.g. Prochaska Wolfe 2002), because (a) the damping wings of the Lya line are very sensitive to the column density of H I (b) several atomic transitions are often available for elements of interest and (c) ionisation corrections are normally small, because the gas is mostly neutral and the major ionisation stages are observed directly (Vladilo et al. 2001). Dust depletions can be a complication, but even these are not as severe in DLAs as in the local interstellar medium (Pettini et al. 1997a) and can be accounted for with careful analyses (e.g. Vladilo 2002a). Thus, abundance... 

The interstellar medium constitutes 10 % of the mass of the galaxy. It can be subdivided into environments with very low-density hot gas, environments with warm intercloud gas, and regions with denser and colder material (23). H and He gas are the major components of interstellar clouds molecules and submicron dust particles are only present in small concentration (22). Through gas phase reactions and solid-state chemistry, gas-grain interactions can build up complex organic molecules. Silicate and carbon-based micron-sized dust particles provide a catalytic surface for a variety of reactions when they are dispersed in dense molecular clouds 24). In cold clouds such dust particles... 

---