Interstellar dust elemental abundances

My more prosaic approach is to consider the elements one by one. For each chemical element I first introduce some properties, perhaps chemical, perhaps poetic, perhaps cultural. Few today know the elements, and fewer can choose to read a chemistry textbook to find out. Each elementintroduction is followed by an account, isotope by isotope, of the isotopic abundance and its measured variations, how these may be accounted for on the basis of the nucleosynthesis theory, and of the cosmochemical implications for interstellar dust and for the origin of the solar system. These have inspired my scientific life. So penetrating are their clues that the proliferation ofisotopic connections smacks of a hard rain on the face - bracing, daunting, overwhelming, refreshing. That is how I want the reader to experience the isotopes, because that is what they are to me. 

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. 

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... 

Cosmic abundances in the interstellar medium are derived by measuring elemental abundances in stellar photospheres, the atmospheric layer just above the stellar surface. Such measurements indicate the amount of elements available for the formation of molecules and particles. Cosmic dust models indicate that up to 80% of the carbon in the photon-dominated diffuse interstellar medium is incorporated into solid aromatic macromolecules and gaseous polycylic aromatic hydrocarbons (41,30). CO gas and C-based ice species (such as CO, CO2, CH3OH and others) may be responsible for up to -25 % of the carbon in cold dense interstellar regions. 

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 presence of dust in DLAs can be inferred by comparing the gas phase abundances of two elements which in local interstellar clouds are depleted by differing amounts. The [Cr/Zn] ratio is one of the most suitable of such pairs for the reasons described above. It became apparent from the earliest abundance measurements in DLAs that this ratio is generally sub-solar, as expected if a fraction of the Cr has been incorporated into dust grains. Figure 10 shows this result for a subset of the DLAs in Figure 7 similar plots are now available for larger samples of DLAs and for other pairs of elements, one of which is refractory and the other is not (e.g. Prochaska Wolfe 1999 2002). 

Below we will see that meteorites, smaller rocks from asteroidal objects delivered to Earth, provide important information for solar system abundances of non-volatile elements. Other sources to refine solar system abundances are analysis of other solar system objects such as the gas-giant planets, comets and the interplanetary dust particles from comets. Outside the solar system, the compositions of hot B stars, planetary nebulae, Galactic cosmic rays (GCR), the nearby interstellar medium (ISM) and H II regions have been employed to amend the solar system abundances of some elements. 

The cycle of birth and death of stars that is initiated by population III stars constantly increases the abundance of heavy elements in the interstellar medium, a crucial prerequisite for terrestrial (rocky) planet formation and subsequently for the origin of life (75). Metals dispersed in the interstellar gas or incorporated into micron-sized dust particles and molecules like CO and water, have the ability to cool the interstellar gas much more efficiently than molecular hydrogen does for population III stars. These elements and molecules are also excited through atomic and molecular collisions and their return to lower lying energy levels releases energy via far-infrared and sub-millimeter radiation below... 

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