Cosmic abundances

Percentage of meteorites seen to fall. Chondrites. Over 90% of meteorites that are observed to fall out of the sky are classified as chondrites, samples that are distinguished from terrestrial rocks in many ways (3). One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Elemental composition is also a property that distinguishes chondrites from all other terrestrial and extraterrestrial samples. Chondrites basically have undifferentiated elemental compositions for most nonvolatile elements and match solar abundances except for moderately volatile elements. The most compositionaHy primitive chondrites are members of the type 1 carbonaceous (Cl) class. The analyses of the small number of existing samples of this rare class most closely match estimates of solar compositions (5) and in fact are primary source solar or cosmic abundances data for the elements that cannot be accurately determined by analysis of lines in the solar spectmm (Table 2). Table 2. Solar System Abundances of the Elements ... 

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

G. G. Goles, "Cosmic Abundances," in ELandbook of Geochemistry, Vol. 1, Springer-Vedag, Berlin, New York, 1969. 

There is a very low cosmic abundance of boron, but its occurrence at all is surprising for two reasons. First, boron s isotopes are not involved in a star s normal chain of thermonuclear reactions, and second, boron should not survive a star s extreme thermal condition. The formation of boron has been proposed to arise predominantly from cosmic ray bombardment of interstellar gas in a process called spallation (1). 

Figure 1.1 Cosmic abundances of the elements as a function of atomic number Z. Abundances are expressed as numbers of atoms per 10 atoms of Si and are plotted on a logarithmic scale. (From A. G. W. Cameron, Space Sci. Rev. 15, 121-46 (1973), with some updating.)...

S2-4 Helium burning as additional process for nucleogenesis 19S4 Slow neutron absorption added to stellar reactions 195S-7 Comprehensive theory of stellar synthesis of all elements in observed cosmic abundances 196S 2.7 K radiation detected... 

Schematic representation uf the main features of the curve of cosmic abundances shown in Fig. 1.1, labelled according tn the various stellar reactions considered to be re.sponsible for the synthesis of the elements. (After E. M. Burbidge et...

One of the most obvious features of Figs. 1.1 and 1.5 is the very low cosmic abundance of the stable isotopes of lithium, beryllium and... 

The nuclei of iron are especially stable, giving it a comparatively high cosmic abundance (Chap. 1, p. 11), and it is thought to be the main constituent of the earth s core (which has a radius of approximately 3500 km, i.e. 2150 miles) as well as being the major component of siderite meteorites. About 0.5% of the lunar soil is now known to be metallic iron and, since on average this soil is 10 m deep, there must be 10 tonnes of iron on the moon s surface. In the earth s crustal rocks (6.2%, i.e. 62000ppm) it is the fourth most abundant element (after oxygen, silicon and aluminium) and the second most abundant metal. It is also widely distributed. 

FIGURE 14.1 These charts show the relative abundances of the principal elements in (a) the universe (the "cosmic abundances") (b) the crust of the Earth and (ci the human hody... 

FIGURE 17.11 The variation in cosmic nuclear abundance with atomic number. Note that elements with even atomic numbers (brown curve) are consistently more abundant than neighboring elements with odd atomic numbers (blue curve). 

The nuclei of some elements are stable, but others decay the moment they are formed. Is there a pattern to the stabilities and instabilities of nuclei The existence of a pattern would allow us to make predictions about the modes of nuclear decay. One clue is that elements with even atomic numbers are consistently more abundant than neighboring elements with odd atomic numbers. We can see this difference in Fig. 17.11, which is a plot of the cosmic abundance of the elements against atomic number. The same pattern occurs on Earth. Of the eight elements present as 1% or more of the mass of the Earth, only one, aluminum, has an odd atomic number. 

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 most abundant isotope is which constitutes almost 99% of the carbon in nature. About 1% of the carbon atoms are There are, however, small but significant differences in the relative abundance of the carbon isotopes in different carbon reservoirs. The differences in isotopic composition have proven to be an important tool when estimating exchange rates between the reservoirs. Isotopic variations are caused by fractionation processes (discussed below) and, for C, radioactive decay. Formation of takes place only in the upper atmosphere where neutrons generated by cosmic radiation react with nitrogen ... 

Theoretical models for nucleosynthesis in asymptotic giant branch stars predict a large contribution to the cosmic nitrogen abundance from intermediate-mass stars [1], In particular, hot-bottom-burning in stars above a certain mass produces [C/N] —1 [2]. However, observations of C and N abundances in C-rich, metal-poor stars, usually using the CH and CN bands, show [C/N] values that vary between —0.5 and 1.5. (Fig. 1). If any of these stars have been polluted by intermediate mass AGB stars, then they should have lower [C/N] ratios. However, most of the CH stars with detailed abundances have [C/Fe] > 1.0, and it is more likely than stars mildly enhanced in C have been polluted by N-rich stars. 

Theory doesn t tell us what initial Li a star has, only what depletion it suffers. An accurate estimate of the initial Li abundance is therefore a pre-requisite before observations and models can be compared. The Sun is a unique exception, where we know the present abundance, A(Li) = 1.1 0.1 (where A(Li)= log[AT(Li)/AT(H)] + 12) and the initial abundance of A(Li)= 3.34 is obtained from meteorites. For recently born stars, the initial Li abundance is estimated from photospheric measurements in young T-Tauri stars, or from the hotter F stars of slightly older clusters, where theory suggests that no Li depletion can yet have taken place. Results vary from 3.0 < A(Li) < 3.4, somewhat dependent on assumed atmospheres, NLTE corrections and TeS scales [23,33]. It is of course quite possible that the initial Li, like Fe abundances in the solar neighbourhood, shows some cosmic scatter. Present observations certainly cannot rule this out, leading to about a 0.2 dex systematic uncertainty when comparing observations with Li depletion predictions. 

M. Asplund, N. Grevesse, A. Jacques Sauval The solar chemical composition . In Cosmic abundances as records of stellar evolution and nucleosynthesis, ed. by F.N. Bash, T.G. Barnes (ASP San Francisco 2005), in press... 

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. 

McWilliam, Smecker-Hane, Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis ASP Conference Series, Ed. Bash and Barnes, (2005)... 

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