Carbon-rich particles

Figure 13 A 5 jjim interplanetary dust particle. This is a carbon-rich particle with chondritic elemental composition. It is porous and entirely composed of anhydrous phases. This 10 ° g particle is an aggregate of >10 umelated and unequilibrated grains. The smooth grains are t5fpically single mineral grains such as Fo, En, or pyrrhotite or carbonaceous material, and the <0.5 p,m lumpy grains are usually GEMSs. This is a relatively t5fpical example of the particles that have entry speeds consistent with cometary origin.

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

Figure 2 shows the SEM image of the flake at a magnification of 350 x, as it was mounted on the conductive carbon tape. If there is a polymeric film covering the sample, the SEM will only show the surface topography of the film, not the structure residing below the polymeric film. EDS was conducted on two areas on the sample as indicated in Figure 2. The EDS analysis was conducted in square spot mode, approximately 1 pm by 1 pm in size. The elemental results are shown in Table 2. Based on these data sets it is apparent that the Type A defect is an iron-rich particle. Based on the lack of chromium or nickel the Type A defect is a particle of steel, not stainless steel.

Reference materials that represent the primary deep-sea and coastal depositional environments and biological materials would solve many of the problems that radiochemists face in analysis of sediments from these settings. Radiochemists require reference materials comprising the primary end member sediment and biological types (calcium carbonate, opal, and red clay from the deep-sea and carbonate-rich, silicate-rich, and clay mineral-rich sediments from coastal environments and representative biological materials). Additional sediment reference material from a river delta would be valuable to test the release of radionuclides that occurs as riverine particles contact seawater. 

While we have direct observational evidence for small molecules and large grains, we have very little information about clusters in the outflows from carbon-rich stars. That is, we know very little about the particles with more than 10 atoms, but, say, fewer than 10 atoms in circumstellar regions (see Kroto Jura 1992). 

To date, neither PAH emission nor absorption has been detected in the circumstellar envelope around a cool carbon star PAH emission has only been seen in carbon-rich environments where there is substantial energy density of ultraviolet radiation. This correlation could simply be an excitation effect the carbon features are only excited by the presence of ultraviolet radiation. However, it could also be that carbon particles are eroded into PAHs in the environment where ultraviolet penetrates either directly by the ultraviolet radiation or indirectly by shocks that accompany the radiation. 

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. 

Based on the bulk chemistry, IDPs are divided into two groups (i) micrometer-sized chondritic particles and (ii) micrometer-sized nonchondritic particles. A particle is defined as chondritic when magnesium, aluminum, silicon, sulfur, calcium, titanium, chromium, manganese, iron, and nickel occur in relative proportions similar (within a factor of 2) to their solar element abundances, as represented by the Cl carbonaceous chondrite composition (Brownlee et al., 1976). Chondritic IDPs differ significantly in form and texture from the components of known carbonaceous chondrite groups and are highly enriched in carbon relative to the most carbon-rich Cl carbonaceous chondrites (Rietmeijer, 1992 Thomas et al., 1996 Rietmeijer, 1998, 2002). 

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