Spite plateau

Fig. 8.3. Lithium, beryllium and iron. The symbol [Fe/H] denotes the logarithm of the ratio of Fe/H for the star and Fe/H for the Sun. The evolution of lithium and beryUium in the halo [Fe/H] < — 1 is a classic example. The lithium content remains independent of the iron content in halo stars. This is known as the Spite plateau, named after the two French astronomers Monica and Fran ois Spite. It indicates a primordial origin (i.e. in the Big Bang). An upturn occurs just when the disk stars begin to take over. Berylhumis an archetypal example of elements created by spallation. Its abundance increases monotonicaUy by accumulation as time goes by.

Figure 8. A compilation of the lithium abundance data from stellar observations as a function of metallicity. N(Li) = 1012(Li/H) and [Fe/H] is the usual metallicity relative to solar. Note the Spite Plateau in Li/H for [Fe/H] — 2.

In Pinsonneault et al. (2002) we adopted for our baseline (Spite Plateau) estimate [Li]= 2.2 0.1 for an overall depletion factor we chose 0.2 0.1 dex. Combining these linearly, we derived an estimate of the primordial lithium abundance of [Li]p = 2.4 0.2. I will use this in the comparison between theory and observation to be addressed next. 

Here, too, the spread in the level of the Spite Plateau dominates the formal errors in the means among the different data sets. To this must be added the uncertainties due to temperature scale and model atmospheres, as well as some allowance for dilution or depletion over the long lifetimes of the metal-poor halo stars. Attempting to accomodate all these sources of systematic uncertainty, I adopt the Pinsonneault et al. (2002) choice of [Li]P = 2.4 0.2. 

What is the level of the Spite Plateau lithium abundance Which observations can pin down the systematic corrections due to model stellar atmospheres and temperature scales and which may reveal evidence for, and quantify, early-Galaxy production as well as stellar depletion/destruction ... 

The Big Bang. In what is generally known as the standard family of Big Bang (Friedmann) models, 7Li is the only LiBeB nuclide synthesised in observable amounts. This Li in full or in part is seen in warm very metal-poor stars, as the Spite plateau. Nonstandard Big Bang models in a wide variety of forms have been proposed. Often, the consequences for the primordial nucleosynthesis are a focus of these proposals. 

Main sequence stars not red giants must be used to map the rise of the Galactic Li abundance from the Spite plateau at low metallicities to its present value in interstellar gas and young stars. Red giants must be discarded because their convective envelope reduces the surface Li abundance below its initial value by an amount that is not yet predictable quantitatively with certainty. Even many main sequence stars have a Li abundance below their initial value. We say lithium has been astrated. Various proposals for astration in main sequence stars have been made but none are developed to the quantitative level that would allow an astrated/reduced Li abundance to be revised upward with the precision desired. Therefore, the upper envelope in a plot of Li/H vs Fe/H is taken to define the Galactic evolution. This shows that the Li abundance remains close to its Spite plateau for [Fe/H] < —1.5 but then rises to about loge(Li) = 3.3 at [Fe/H] = 0. 

II. Pop I stars are late stars which include the current generation of stars. Pop II stars are older and Pop III refer to the very first generation of stars. Observations have long shown [53, 54, 55] that Li does not vary significantly in Pop II stars with metallicities < 1/30 of solar — the Spite plateau . Recent precision data... 

One of the major cosmological developments of the 1980s was the discovery of the Li plateau in low metallicity halo stars (Spite and Spite 1982, see Fig. 1 top left panel). The unique behaviour of that element, i.e. the constancy of the Li/H ratio with metallicity, strongly suggests a primordial origin. The observed... 

That the Li abundance observed by Spite and Spite (1982) could not be the cosmological abundance but had been reduced by a factor of at least 4 by either diffusion or burning was first noted by Michaud, Fontaine and Beaudet (1984). These authors also emphasized that this plateau is constant over a surprisingly large Teff interval. This remains a problem requiring further study. 

It is interesting to look at the temperature dependence of nSSA. The results are shown in Fig. 52, in which PMMA data are also plotted. Whereas PMMA and MGIM76 have plateau values, nSSA0, that are rather close, there is a huge increase with CMIM25. Furthermore, in spite of the same Ta values, the plateau ends up at about 80 °C for CMIM25 instead of 115 °C for MGIM76. 

One interesting aspect is the comparison of these highly symmetric dendrimers with 3G, which has a compositional similarity to the symmetrical phenylenes. Since the Tg of 3 is invariant with molecular weight, we believe that its molecular weight is in a plateau region where the Tg does not increase with the molecular weight. In spite of this, it seems... 

Another way of determining whether the observed plateau does really correspond to [M]e is to perform polymerization at various ratios of [M to [I]0. The properly determined value of [M]e should be independent of this ratio, provided that [M]o/[I]o is not too small. Thus, for non-living systems it is necessary to carry out polymerizations with increasing initial initiator concentration until a constant ultimate monomer conversion is reached. This method of approaching the equilibrium concentration gave reliable thermodynamic parameters for the cationic polymerization of cyclic esters of phosphoric acid, in spite of termination observed in these systems 11 ... 

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