3.4. Lithium-7

Lithium-7 is fragile, burning in stars at a relatively low temperature. As a result, the majority of interstellar ⁷Li cycled through stars is destroyed. For the same reason, it is difficult for stars to create new ⁷Li and return it to the ISM before it is destroyed by nuclear burning. As the data in Figure 8 reveal, only relatively late in the evolution of the Galaxy, when the metallicity approaches solar, does the lithium abundance increase noticeably. However, the intermediate-mass nuclides ⁶Li, ⁷Li, ⁹Be, ¹⁰B, and ¹¹B can be synthesized via Cosmic Ray Nucleosynthesis (CRN), either by alpha-alpha fusion reactions, or by spallation reactions (nuclear breakup) between protons and alpha particles on the one hand and CNO nuclei on the other. In the early Galaxy, when the metallicity is low, the post-BBN production of lithium is expected to be subdominant to the pregalactic, BBN abundance. This is confirmed in Figure 8 by the "Spite Plateau" (Spite & Spite 1982), the absence of a significant slope in the Li/H versus [Fe/H] relation at low metallicity. This plateau is a clear signal of the primordial origin of the low-metallicity lithium abundance. Notice, also, the enormous spread among the lithium abundances at higher metallicity. This range in Li/H results from the destruction/dilution of lithium on the surfaces of the observed stars, implying that it is the upper envelope of the Li/H versus [Fe/H] relation which preserves the history of the Galactic lithium evolution. Note, also, that at low metallicity this dispersion is much narrower, suggesting that the corrections for depletion/dilution are much smaller for the Pop II stars.

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.

As with the other relic nuclides, the dominant uncertainties in estimating the primordial abundance of ⁷Li are not statistical, they are systematic. Lithium is observed in the atmospheres of cool stars (see Lambert (2002) in these lectures). It is the metal-poor, Pop II halo stars that are of direct relevance for the BBN abundance of ⁷Li. Uncertainties in the lithium equivalent width measurements, in the temperature scales for these Pop II stars, and in their model atmospheres, dominate the overall error budget. For example, Ryan et al. (2000), using the Ryan, Norris & Beers (1999) data, infer [Li]_P 12 + log(Li/H)= 2.1, while Bonifacio & Molaro (1997) and Bonifacio, Molaro & Pasquini (1997) derive [Li]_P = 2.2, and Thorburn (1994) finds [Li]_P = 2.3. From recent observations of stars in a metal-poor globular cluster, Bonifacio et al. (2002) derive [Li]_P = 2.34 ± 0.056. But, there's more.

The very metal-poor halo stars used to define the lithium plateau are very old. They have had the most time to disturb the prestellar lithium which may survive in their cooler, outer layers. Mixing of these outer layers with the hotter interior where lithium has been destroyed will dilute the surface abundance. Pinsonneault et al. (1999, 2002) have shown that rotational mixing may decrease the surface abundance of lithium in these Pop II stars by 0.1 - 0.3 dex while maintaining a rather narrow dispersion among their abundances (see also, Chaboyer et al. 1992; Theado & Vauclair 2001, Salaris & Weiss 2002).

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.