PRIMORDIAL ABUNDANCES AND COSMOLOGICAL IMPLICATIONS

The importance of lithium observations lies on being a probe of cosmological models and also as a tracer for stellar structure and evolution. New measurements of the lithium abundance in a sample of 37 field dwarfs of low metallicity was corried out. Extremely Metal-Deficient (EMD) dwarfs form a much more homogeneous population with regard to ⁷Li abundance that stars somewhat more metal-rich. These EMD dwarfs show Li abundances for surface temperatures between 5500K and 6300K, which average log N(Li)=2.08 (±0.10). A trend towards 2.2 is found as T_eff approaches 6300K. By comparing the Li-T_eff of the EMD stars with the published curve for the Hyades, the astronomers show that it is improbable that the warmer EMD stars have depleted their Li significantly, in spite of their age. They consider that these stars offer the best opportunity to make a reliable measurement of the primordial Li abundance, which they estimate to be log N(Li)=2.2(±0.15).

For somewhat less metal-deficient stars they find a wider scatter in Li abundances. Li depletion appears to set in at higher temperatures for the moderately metal-deficient stars than for the extremely metal-deficient, consistent with metallicity-dependent depletion rates.

Using a recent theoretical curve of Li against the universal baryon to photon ratio from a standard Big Bnag model, and the above cited primordial abundance, n is constrained to the range 1.2×10^-10 <= n <= 8×10^-10.

⁹Be has a special place in schemes of nucleosynthesis. It is the lightest stable nuclide not synthesized in the Big Bang and it is destroyed in stellar interiors at temperatures higher than 3×10⁶K. Nevertheless it has been widely observed in nature: in the sun, in meteorites, in the interstellar medium and in a variety of atmospheres of stars of different ages. The currently accepted explanation for its observed existence is spallation of heavy nuclei, mostly CNO nuclei, ny cosmic rays in the interstellar medium.

Recently six highly metal-deficient dwarfs in the spectral range containing the resonance doublet of ⁹Be II at 3130 Å were observed. Using the method of spectral synthesis astronomers evaluated the abundance of 9Be in these objects. They set upper limits to the ⁹Be abundance, ⁹Be/H, below 2.5×10^-12. These are the first reported detections of ⁹Be in stars with such low metalicities. From these results and literature values for ⁹Be in more metallic stars, they can constrain models for galatic evolution: those with entail an early burst of ⁹Be production are ruled out. The data are used to predict the spallogenic content of ⁷Li, ⁶Li, ¹⁰B and ¹¹B in the material from which the sample stars was formed and hence to limit the galactocentric fraction of ⁷Li in highly metal-deficient stars. A firm upper limit of 0.1 dex can be set to the fraction of measured ⁷Li in such stars which is either non-primordial, or is due to spectral blending of ⁶Li, leaving virtually all of the measured ⁷Li as primordial.