Abundances in the interstellar medium

Show that, in the Simple model (no inflow and no Galactic wind), the evolution of deuterium abundance in the interstellar medium is given by... 

Fig. 12.11. Top panel abundances in the interstellar medium of cB58. Lower panel abundances of the corresponding elements in diffuse clouds of the Milky Way and the same, shifted down by 0.4 dex, to facilitate comparison with cB58. After Pettini el at. (2002b).

It is known that the oxygen abundance in the interstellar medium increases all the time this nucleus is produced by type 11 supernovas which, one after the other, also contribute their iron production to the Galaxy (Fig. 8.7). The pO mechanism is thus likely to grow in importance as the Galaxy evolves. In other words, clues to the Op mechanism should be sought in the early phases of galactic evolution, that is, in halo stars. The fact remains that the two mechanisms induce different evolution in beryllium and boron as a function of oxygen. 

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. 

Among many the nitriles produced, the cyanopolyynes, HC N, are particularly interesting due to their linear conjugated structures, their biological importance, and their abundance in the interstellar medium. The well-known interstellar cyanopolyynes HC N ( = 1,3,5,7) [142] can be formed abundantly in the present system HCN (relative intensity, 43), HCCCN (51), HCCCCCN (46), and HCCCCCCCN (19). In this study, the even numbers of the HC N series (with n at 2,4, and 6) were also detected however, their relative intensities were much less than those of the cyanopolyynes with odd carbon numbers. 

Another measure of the cosmic ray ionization rate would be provided by the observed HD abundances, if the overall deuterium abundance D]/[HJ in interstellar clouds were known. Alternatively, the values of (o derived from the oxygen chemistry can be used to infer DJ/ H from the measured HD column densities. The derived deuterium abundances in the models of vDB are in the range (0.5-2.0)xl0 for the various clouds, consistent with other estimates of the deuterium abundance in the interstellar medium, [D]/[H]=( 1.5 1.0) X 10" (Vidal-Madjar and Gry 1984). The models of the f Per cloud favor the upper part of this range, whereas those of the ( Oph cloud give somewhat lower values, due to the order of magnitude lower HD column density. A similar deuterium abundance for the ( Oph cloud has been obtained by VRA. 

The hydrogen atom and its spectrum are of enormous importance in astrophysics because of the large abundance of hydrogen atoms both in stars, including the sun, and in the interstellar medium. 

The existence and distribution of the chemical elements and their isotopes is a consequence of nuclear processes that have taken place in the past in the Big Bang and subsequently in stars and in the interstellar medium (ISM) where they are still ongoing. These processes are studied theoretically, experimentally and obser-vationally. Theories of cosmology, stellar evolution and interstellar processes are involved, as are laboratory investigations of nuclear and particle physics, cosmo-chemical studies of elemental and isotopic abundances in the Earth and meteorites and astronomical observations of the physical nature and chemical composition of stars, galaxies and the interstellar medium. 

Clouds of gas in the interstellar medium are called gaseous nebulas. These nebulas are regions of the interstellar medium with above-average density. The proportions of elements in the interstellar medium conform to the abundances in the table, that is, 90% hydrogen atoms, 9% helium atoms and less than 1% heavier atoms, where these percentages now refer to relative numbers of atoms rather than relative mass. 

Everything suggests that the Fe/H ratio can be taken as a kind of chronometer, or at least as an index of evolution. It defines the chemical history of the Galaxy, and cannot decrease. The accumulation of iron in the interstellar medium is such that the abundance of this element increases monotonically, although in a way that is far from linear. The Fe/H ratio can be calibrated as a function of time by jointly determining the iron content and age of a great many stars selected from distinct generations. This then constitutes the basis of the age-metallicity relation. 

Model calculations that include at least some of the reactions we have discussed for the syntheses of complex molecules have been performed in the last several years. Both steady-state and chemical time dependent models have been published. Unfortunately, as models include more and more complex species, they become more and more sensitive in their predictions to small changes. As an example, consider two models that in their predictions of the abundances of one-carbon-atom hydrocarbons differ by a factor of 3. This factor is not considered to be a major one in the field of interstellar chemistry. However, since the two-carbon-atom hydrocarbons are formed by reactions between one-carbon atom species, the model will differ in their predictions for the abundances of the larger hydrocarbons by a factor of 9. As one can easily discern, the situation becomes worse as the size of the hydrocarbons increase. Given this extreme sensitivity, modelers should attempt to make sure that at each stage of molecular complexity, they consider all depletion mechanisms and do not overestimate the abundances of the complex molecules that are intermediates in the formation of still more complex species. Unless this is done, models can become in our view overly optimistic about the growth of complexity in the interstellar medium. 

Again because of its high volatility, sulfur is relatively undepleted from interstellar gas by condensation in interstellar grains, so that its isotopic composition has been pardy measured by radioastronomers, who detect emission lines from the molecule CS in the interstellar medium. By comparing the rotational transitions of 13C/32S with those of 12C/34S the interstellar isotopic ratio has been found for sulfur (see 34S, Anomalous isotopic abundance). 

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. 

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. 

Wasserburg G. J., Busso M., and Gallino R. (1996) Abundances of actinides and short-lived nonactinides in the interstellar medium diverse supernova sources for the r-processes. Astrophys. J. 466, L109-L113. 

Table 9.3. Comparison of the abundance of ices in the interstellar medium (towards IRS9) and of cometaiy volatiles (at l AU) [4]...

The popular at present scenarios which explain the cosmic ray source composition include the acceleration of grains together with relatively less abundant volatile ions by shocks in the interstellar medium [19], the acceleration of freshly formed material particularly grains in young supernova remnants [20], and the acceleration of material in hot superbubbles with multiple supernova remnants [21],... 

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