3.3. Lithium-7

Cosmologically interesting lithium is observed in the spectra of nearly 100 very metal-poor halo stars [17, 18]. These stars are so metal-poor they provide a sample of more nearly primordial material than anything observed anywhere else in the Universe; the most metal-poor among them have heavy element abundances less than one-thousandth of the solar metallicity. However, these halo stars are also the oldest objects in the Galaxy and, as such, have had the most time to modify their surface abundances. So, even though any correction for Galactic evolution modifying their lithium abundances may be smaller than the statistical uncertainties of a given measurement, the systematic uncertainty associated with the dilution and/or destruction of surface lithium in these very old stars could dominate the error budget. There could be additional errors associated with the modeling of the surface layers of these cool, low-metallicity, low-mass stars needed to derive abundances from absoprtion-line spectra. It is also possible that some of the Li observed in these stars is non-primordial (e.g., that some of the observed Li may have been produced post-BBN by spallation reactions (the breakup of C, N, and O nuclei into nuclei of Li, Be, and B) or fusion reactions ( + to form ⁶Li and ⁷Li) in cosmic-ray collisions with gas in the ISM). In a recent analysis [20], it is argued that as much as ~ 0.2 dex of the observed lithium abundance, A(Li) 12 + log(Li/H), could be post-primordial in origin.

The very large data set of lithium abundances measured in the warmer (T > 5800 K), more metal-poor ([Fe/H] < -1.3) halo stars define a plateau (the``Spite-plateau'' [17]) in the lithium abundance - metallicity plane. Depending on the choice of stellar-temperature scale and stellar atmosphere model, the abundance level of the plateau is: A(Li) = 2.2 ± 0.1, with very little dispersion in abundances around this plateau value. The small dispersion provides an important constraint on models which attempt to connect the present surface lithium abundances in these stars to the original lithium abundance in the gas out of which these stars were formed some 10 - 15 Gyr ago. ``Standard'' (i.e., non-rotating) stellar models predict almost no lithium depletion and, therefore, are consistent with no dispersion about the Spite-plateau. Although early work on mixing in models of rotating stars was very uncertain, recently stellar models have been constructed which reproduce the angular momentum evolution observed for the much younger, low-mass open cluster stars. These models have been applied to the study of lithium depletion in main sequence halo stars. A well-defined lithium plateau with modest scatter and a small population of ``outliers'' (overdepleted stars), consistent with the current data, is predicted for depletion factors between 0.2 dex and 0.4 dex [19].

To err on the side of caution, a generous range for the plateau abundance, 2.1 A(Li) 2.3, is adopted. If depletion is absent, this range is consistent with the primordial lithium ``valley'' (see Fig. 4). For depletion 0.2 dex, the consistent primordial lithium abundances bifurcate and move up into the ``foothills'', although a non-negligible contribution from post-BBN lithium could move the primordial abundance back down again (Fig. 4).