8.1. Primordial ⁷Li

Ryan, Norris and Beers [113] conclude ⁷Li / H 10^-10, with small random errors and three sources of systematic error, each up to a factor of 1.3, from the effective temperatures, stellar atmospheres and enhancement prior to star formation. Bonifacio & Molaro [140] found ⁷Li / H = 1.73 ± 0.05 ± 0.2 x 10^-10. These abundances are both below the value of 3.5^+1.1_-0.9 x 10^-10 (95%)�� from BBN and our D/H, but unlike [113], we feel they are not inconsistent given the quoted systematic errors, the lack of understanding of depletion, and the variation amongst similar stars. We do not know how to estimate the systematic errors connected with these issues. Given the comparative simplicity of D/H, we prefer to use it and SBBN, and we stick with our earlier suggestion [53] that ⁷Li in the Spite plateau halo stars is depleted by about a factor of two. Most, but not all agree that this is reasonable. Depletion by much larger factors, which was discussed a few years back, is now our of favor because of improved models. Improved modelling of rotational mixing, has lead to better fits to high metal abundance (population I) stars, which can be applied to halo (population II) stars, while the initial rotation rates of the halo stars may be lower than was assumed (Deliyannis private communication).

In summary, both the data and theory tells us that the ⁷Li on the Spite plateau is not exactly the primordial value. The correction is probably small, less than a factor of two, but we do not yet know its value.

If we are to attain a primordial ⁷Li abundance we must either (1) understand why its abundance varies from star to star, and learn to make quantitative predictions of the level of depletion, or (2) make measurements in relatively unprocessed gas.

We are optimistic that primordial ⁷Li will be measured to high precision. Compared to D and He, the observations are simple: 15-20 mÅ absorption lines in relatively empty spectra of often bright stars (V = 11). The best data have small errors. We anticipate that further studies will determine the amount of ⁷Li produced prior to the formation of the stars, and the subsequent depletion in these stars. The possible increase in ⁷Li with iron abundance is a clue, as are the ⁶Li, Be and B abundances in the same stars.