Interstellar medium metallicity

Fig. 9.8. Trend of beryllium abundance with metallicity compared to predictions from two models (a) CRS denotes cosmic-ray acceleration in superbubbles rich in iron and oxygen as predicted from theoretical supernova yields (in this case those of Tsujimoto and Shigeyama 1998) and (b) CRI denoting cosmic rays accelerated from the general interstellar medium. The density dependence comes from its influence on the delay in the deposition of the synthesized Be. Virtually identical results were obtained using the yields from Woosley and Weaver (1995). After Ramaty et al. (2000).

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. 

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. 

The model assumes that evolution takes place in a closed system, with successive generations of stars being bom into the interstellar medium. At each generation, a fraction of the gas is transformed into metals and returned to the interstellar medium. Gases imprisoned in stars of low mass and compact residues play no further role in galactic evolution. In this model, metaUicity is bound to increase as time goes by. And so the arrow of galactic time is defined. Evolution will continue until no further gas is available to form new stars. 

These metals, together with the pristine stellar material is restored into the interstellar medium (ISM) at the star death. This process clearly affects crucially the chemical evolution of the ISM. In order to take into account the elemental production by stars we define the yields , in particular the stellar yields (the amount of elements produced by a single star) and the yields per stellar generation (the amount of elements produced by an entire stellar generation). 

From this body of data it is now firmly established that the depletions of refractory elements are generally lower in DLAs than in interstellar clouds of similar column density in the disk of the Milky Way. The reasons for this are not entirely clear. The question has not yet been addressed quantitatively qualitatively the effect is probably related to the lower metallicities of the DLAs and the likely higher temperature of the interstellar medium in these absorbers (Wolfire et al. 1995 Petitjean, Srianand, Ledoux 2000 Kanekar Chengalur 2001). Figure 10 does seem to indicate a weak trend of decreasing Cr depletion with decreasing metallicity, also supported by the results of Prochaska Wolfe (2002). 

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... 

Cosmic rays [...] come from the matter in what are known as superbubbles, tremendous cavities blasted out of the interstellar medium by the winds of giant stars and the explosions of supernovae. Typically, a superbubble is formed over millions of years, not by one star, but by the winds and supernovae of many. The explosions fill the superbubbles with hot, rarefied plasma, or in other words ionized gases. Compared with the surrounding interstellar medium, the gases in superbubbles are rich in metals (sic) such as carbon and oxygen, which are cooked up inside the stars and in the supernovae that created the superbubbles in the first place. 

The interaction of aromatic hydrocarbons with iron cations has been studied in the gas phase. The results of these studies provide information useful in the proposal that polycyclic aromatic hydrocarbons undergo efficient reactions with iron and other transition metals in the interstellar medium. Fe(r/-G2H4)2(r7-PhMe) 87 is a useful precursor for a range of arene iron complexes. Thus, reaction of 87 with the desired naphthalene derivative and three molecules of butyne or hexyne leads to the (hexaalkylbenzene)naphthalene iron complexes, as shown in Scheme 15. ... 

Analyses of this kind have shown that nearly all stars have very similar elemental compositions. The spectra of some stars, however, reveal much lower abundances than in the Sun. These so-called subdwarfs are metal-poor by factors up to 500. These compositional variations are correlated with the mass, age and dynamics of the stars within the Milky Way. The stars that formed first have the lowest elemental abundances. As those stars evolved - the more massive ones more rapidly than the less massive ones - they polluted the interstellar medium with the nucleosynthetic products formed in their interiors either through a gentle wind (low-mass stars) or through a violent supernova explosion (massive stars). In this way, later generations of stars are formed from gas with higher elemental abundances. It is this elemental enrichment that drives the evolution of the Milky Way and other galaxies. 

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