Galaxy isotope abundances

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. 

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. 

Observations of isotopic abundances provides information on the nucleosynthesis operating in the compact core of stars and supernova explosions and on the chemical evolution of the Galaxy. The CNO nuclides in late-type stars are affected by freshly synthesized core material brought up by dredge-up events. On the other hand, the Si isotopes are involved in later phases of nuclear burning, a narrow span of the red giant lifetime before planetary nebulae or supernovae. Therefore relative abundances of Si isotopes we observe remain unchanged from those of interstellar matter from which a star was formed. 

The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed, or usually assumed to be identical to that of the sun. The few exceptions to this include elements and isotopes such as lithium and deuterium that are destroyed in the bulk of the sun s interior by nuclear reactions. The composition of the sun as determined by optical spectroscopy is similar to the majority of stars in our galaxy, and accordingly the relative abundances of the elements in the sun are referred to as "cosmic abundances." Although the cosmic abundance pattern is commonly seen in other stars there are dramatic exceptions, such as stars composed of iron or solid nuclear matter, as in the case with neutron stars. The... 

The primordial Li abundance was sought primarily because of its ability to constrain the baryon to photon ratio in the Universe, or equivalently the baryon contribution to the critical density. In this way, Li was able to complement estimates from 4He, the primordial abundance of which varied only slightly with baryon density. Li also made up for the fact that the other primordial isotopes, 2H (i.e. D) and 3He, were at that time difficult to observe and/or interpret. During the late 1990 s, however, measurements of D in damped Lyman alpha systems (high column-density gas believed to be related to galaxy discs) provided more reliable constraints on the baryon density than Li could do (e.g. [19]). Even more recently, the baryon density has been inferred from the angular power spectrum of the cosmic microwave background radiation, for example from the WMAP measurements [26]. We consider the role of Li plateau observations post WMAP. 

In the final section of this chapter, we discussed the formation of galaxies and the formation and chemical evolution of the Milky Way. This sequence of events set the stage for the formation of the solar system. In Chapter 4, we will look at the resulting abundances of the elements and isotopes, both in the solar system and in the galaxy. The solar system abundances of the elements are a fundamental constraint for understanding the Sun, the planets, and the smaller bodies in the solar system. 

In this chapter, we discuss the abundances of the elements and isotopes in the solar system. First, we look at the techniques used to determine solar system abundances, including spectroscopy of the stellar photosphere, measurements of solar wind, and analyses of chondritic meteorites. The solar system abundances of the elements and isotopes are then presented. These abundances are then compared to the abundances in the solar neighborhood of the galaxy and elsewhere. Finally, we introduce how solar system abundances provide a basis for much of what we do in cosmochemistry. 

In this chapter, we have introduced solar system abundances and cosmic abundances of the elements and isotopes and briefly discussed their importance in cosmochemistry. The idea of cosmic abundances has been around for over a century and is rooted in measurements of many different celestial objects. Only in the last 20-30 years has our knowledge of the compositions of the solar system, of stars and molecular clouds in our galaxy, and of other galaxies grown sufficiently that we can now detect clear compositional differences between our solar system and other objects in the galaxy and the universe. 

The value of the decay constant (1) must have remained constant over the age of the solar system and the galaxy, and it must be accurately known. As we discussed in Chapter 2, this third assumption is well founded for conditions relevant to cosmochemistry. The concordance of dates given by systems using a variety of decay paths and astronomical observations of decay rates of newly synthesized elements over billions of years provides strong evidence that the decay rates have remained constant. In addition, detailed experiments and theoretical models have identified the extreme conditions (e.g. centers of stars) under which this assumption breaks down for certain isotopes, thereby identifying the exceptions that prove the rule. (4) It must be possible to assign a realistic value to the initial abundance of the... 

The abundances of radioactive isotopes over time in the galaxy can be modeled based on the above considerations. With an approximately constant production rate, the abundance of a stable nuclide will grow and will be proportional to the time over which it has been produced. In contrast, the abundance of a radionuclide will reach a steady state between production and decay in about eight mean lifetimes. (We will use mean life (t) instead of... 

We can now construct a model that describes the evolution of the abundances of stable and radioactive isotopes in the gas phase of the galaxy as a function of time. Such as model provides an estimate of the abundances of the radioactive nuclides that should have been available in the average interstellar medium at the time the solar system formed. By comparing these predictions with the abundances inferred for the early solar system from meteorites, we can investigate the environment in which the solar system formed. 

The molecules found to date are composed of the elements H, C, N, O, Si, S, and Cl with the bulk of the molecules containing H, C, N, and O. The light elements H, D, and He are of cosmological origin and are therefore tracers of the early universe. On the other hand the heavier elements C, N, O,... are produced in stars by the processes of stellar nucleosynthesis. In addition to the most abundant isotopic forms many stable isotopes such as D, 13C, 170, lsO, 15N, 30Si, 33S, and 34S have been detected (see Appendix 1). The detailed determination of isotopic ratios — though often beset with formidable difficulties — has become a useful indicator of the chemical evolution of molecular clouds and the past chemical history of the galaxy. 

From the isotopic decomposition of normal oxygen one finds that the mass-17 isotope, 170, is 0.038% ofallO on Earth. It is the least abundant, by a wide margin, of the three stable O isotopes, 2630 times less abundant than the most abundant of the O isotopes, namely l60. This much smaller abundance reflects the history of the Galaxy in an important way namely, it shows that17O cannot be synthesized in stars directly from H and He, as 16 O can be. Using the total abundance of elemental O = 15.2 million per million silicon atoms in solar-system matter, this isotope has... 

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