Primordial abundances

Plutonium occurs in natural ores in such small amounts that separation is impractical. The atomic ratio of plutonium to uranium in uranium ores is less than 1 10 however, traces of primordial plutonium-244 have been isolated from the mineral bastnasite (16). One sample contained 1 x 10 g/g ore, corresponding to a plutonium-244 [14119-34-7] Pu, terrestrial abundance of 7 x 10 to 2.8 x 10 g/g of mineral and to <10g of primordial Pu on earth. The content of plutonium-239 [15117 8-3], Pu, in uranium minerals is given in Table 2. 

Helium is the second most abundant element in the universe (76% H, 23% He) as a result of its synthesis from hydrogen (p. 9) but, being too light to be retained by the earth s gravitational field, all primordial helium has been lost and terrestrial helium, like argon, is the result of radioactive decay ( He from a-decay of heavier elements, " °Ar from electron capture by... 

Neder H, Heusser G, Laubenstein M (2000) Low-level y-ray germanium-spectrometer to measure veiy low primordial radionuclide concentrations. ApplRadiat Isot 53 191-195 Palacz ZA, Freedman PA, Walder AJ (1992) Thorium isotope ratio measurements at high abundance sensitivity using a VG 54-30, an energy-filtered thermal ionization mass spectrometer. Chem Geol 101 157-165... 

The Galactic Bulge contains about 20 percent of the Galaxy s stellar mass. Theories of its formation include a primordial free-fall collapse, remnants of accretion episodes, or secular evolution of bar instabilities ([1]). Accurate stellar abundance determinations can help distinguish between these models. 

As a summary of several studies for light elements in unevolved cluster stars (based on photometric or low dispersion spectroscopic indexes), the right panel of Fig. 1 nicely shows the main features. There are star-to star abundance variations, with large spreads in N anticorrelated with much smaller spreads in C (as found all the way from MS to RGB). The implications are that we are not looking simply at a conversion of C into N, and that primordial inhomogeneities... 

The abundance ratios found in the photospheres of our target stars are imprints of the explosions of the first SNe II or even more massive stars. At very low metallicities there is a reasonable hope that the SNe which have polluted the environment were themselves primordial objects. In former papers on the chemical composition of very metal poor stars, some accent was put on trends of abundance ratios with metallicity ( McWilliam et al.[6], Norris et al. [8]). Such was the case for [Mn/Fe], or [Cr/Fe] decreasing with decreasing metallicity, or... 

Big efforts have been devoted in the last years to the study of light elements abundances. Definitively their importance is strongly related to cosmology as well as to stellar structure and evolution. In fact hints on the primordial nucleosynthesis can be achieved from Li, Be and B primordial abundances. Moreover these studies can be a precious tool for testing and understanding the inner stellar structure, especially for what regards the mixing processes in stellar envelopes [11-... 

From observations of 11 main-sequence stars belonging to the Galactic halo, Spite Spite [27] concluded that the lithium abundance was essentially independent of metallicity for halo stars hotter than 5600 K, and inferred that the Li abundance was hardly altered from the Big Bang. Two decades of work has followed, increasing the number of stars observed and the range of metallicity that they span, in an effort to establish the primordial Li abundance more securely. 

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. 

The difficulties in inferring the primordial Li abundance from halo star observations can be separated into two broad categories ... 

A complication, however, arises from the fact that these anomalies are also due to proton nucleosynthesis (though operating at higher temperatures) and thus that some primordial CNO-abundance anomalies should be expected, too. In fact, 47 Tuc has been for a long time the example for a CN/CH dichotomy all along the cluster sequence, and by now, CN-variations have been found -mainly by Cohen, Briley and coworkers - in unevolved stars of other clusters as well (47 Tuc [3,2], M71 [1], M5 [9],. ..). This complicates the task to identify the purely evolutionary effect due to extra-mixing, which is needed to develop and test physical models. 

In the next section each light nuclide is considered in turn, its post-BBN evolution briefly reviewed along with identification of a few of the potential challenges to accurately inferring the primordial abundances from the observational data. Then, having established that the current data - taken at face value - are not entirely consistent with SBBN, I investigate whether changes in the early universe expansion rate can reconcile them. 

Fig. 4. A summary of the time evolution of primordial 4He abundance determinations (mass fraction Yp) from observations of metal-poor, extragalactic Hu regions (see the text for references). The solid horizontal line is the SBBN-predicted 4He abundance expected for the WMAP (and/or D) inferred baryon density. The two dashed lines show the la uncertainty in this prediction.

The path from acquiring observational data to deriving primordial abundances is long and complex and littered with pitfalls. While the predicted and observed relic abundances are in rough qualitative agreement, at present there exist some... 

Fig. 5. Isoabundance curves for 4He (solid) and D (dashed) in the baryon abundance (7710) - expansion rate factor (S) plane. The labels on the 4He curves are for Yp, while those on the D curves are for j/d = 10B(D/H). The filled circle with error bars corresponds to the adopted values of the D and 4He primordial abundances (see the text).

---