Interstellar medium elements

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. 

Linked to 1) is of course the enrichment of the interstellar medium, to which they are important contributors in nuclearly processed elements as He, C, N, s-elements (Ba etc). Goal 2) can be pursued with nuclearly unprocessed elements , the best accessible of them being O, Ne, Ar and S. 

If an external body is engulfed, it can enrich the star with the original interstellar medium abundances of 6Li, 7Li, 9Be and 10,11B (written here in increasing order of hardness to be destroyed by thermonuclear reactions). This mechanism is then supposed to produce stellar enrichment of these elements up to the maximum meteoritic value. Also, the engulfing star will suffer a rotational increase due to the gain of the planet momentum and a thermal expansion phenomenon due to the penetration of the body provoking mass loss phenomena (Siess Livio 1999). An extension to this scenario has been proposed by Denissenkov Weiss (2000) in order to explain supermeteoritic Li abundance values, via a combination of stellar rotation and activation of the 7Be mechanism at the base of the convective layer produced by the penetration of the external body. 

The interstellar medium is thus a chemically diverse medium fed nearly all of the chemical elements by supernova explosions. Conditions in the interstellar medium produce a cocktail of molecules that ultimately find themselves back on the surface of planets during the formation of the new star and solar system. Does the interstellar medium seed life with molecules from space The nature of interstellar medium chemistry might then add credibility to the formation of life in many places within the Universe and act as a panspermia model for the origins of life. 

Cycle of star formation The collapse of a giant molecular cloud forms a star nuclear synthesis within the star produces more elements the star ages and ultimately dies in a supernova event elements are thrown into the interstellar medium to form a giant molecular cloud. 

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. 

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).

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. 

As a concluding remark of this section, the theoretical models of nucleosynthesis within stars show that the isotopic compositions of the elements are highly variable depending on star size, metallicity, companion s presence. From the isotopic data obtained in diverse solar system materials it turns out that most of this material was highly homogenized in the interstellar medium or by the formation of the solar system. The presence of isotopic anomalies preserved in some primitive materials are the last witnesses of the initial diversity of the materials constituting our planetary system. 

Until 1968, astronomers had always assumed that the interstellar medium was essentially made up of atomic hydrogen. Indeed, this ubiquitous element leaves its trace in every quarter in the form of a specific radiation line at wavelength... 

We owe much to radioastronomy. It has taught us, for example, that the interstellar medium is the site of complex and varied chemistry, quite different to the chemistry we know and practise on Earth. Indeed conditions in space are very special low temperatures and densities are often accompanied by the effects of extreme radiation. All chemistry taking place in space depends on the cosmic abundances of the reagents. The commonest elements taking part in the combinatorial art of atoms are listed in Table 6.1, based on the abundance diagram. 

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. 

It should be noted that odd elements produced in explosive nucleosynthesis depend less on metalhcity than their counterparts fashioned by slow (hydrostatic) nucleosynthesis, for the n/p ratio is steadily modified by various weak interactions operating in the advanced stages. In other words, the n/p ratio deep down in the star, in regions affected by explosive nucleosynthesis, no longer reflects the initial ratio inherited from the interstellar medium. At least, this is what calculations suggest. However, the cause of all these phenomena remains relatively obscure, given the complex way in which nuclear reactions are interwoven within massive stars in the advanced stages of their evolution. 

In recent decades, spectroscopy has revealed that the elemental and isotopic abundances in the galaxy vary with radial position and that the Sun has a somewhat different composition than the molecular clouds and diffuse interstellar medium in the solar neighborhood. For this reason, we can no longer think of the solar system abundances as truly cosmic abundances. 

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... 

In general, the decrease in opacity obtained with the lower abundances characteristic of the LMC tends to help to confine the evolutionary tracks to the blue side of the H-R Diagram. However, most models are computed with the an abundance set taken as solar divided by, say, four. In practice, the LMC abundance distribution is not this simple.Dopita (1986) and Russell, Bessell and Dopita have shown that, in the LMC, the underabundance of various elements with respect to solar is dependent upon their atomic number. For example, Cand N are depleted by about 0.8 dex, O and Ne by about 0.5 dex, Ca by anbout 0.3 dex and the heavy elements from Ti through Fe to Ba by about 0.2 dex. This pattern is similar to that produced in models of deflagration supemovae, and may indicate that these have been relatively more important in enriching the interstellar medium in the LMC. 

They were either formed with the progenitor from the interstellar medium, or they were created in it as an adjunct to helium-burning and mixed to the surface. In the latter event, some enrichment of the elements would be expected, although calculations of the evolution of massive stars have indicated that no products from the helium burning core should be convected to the surface (Lamb et al. 1977). 

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