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3. LiBeB

The question that one should ask with regard to the above discussion of 7Li (and as we will see below when discussing concordance, 7Li will play an important role in determining the baryon density eta), is whether or not there is additional evidence for the post big bang production of 7Li. There is in fact evidence in the related observation of the intermediate mass elements of 6Li, Be and B. While these elements are produced in the big bang [21], their predicted primordial abundance is far below their observed abundance, which like 7Li is determined by observations of old metal poor halo stars. Whereas in the range eta10 = 1.5 - 4.5, standard BBN predicts abundances of

Equation 9 (9)

the observed abundances found in Pop II halo stars are: 6Li/H approx few x 10-12, 9Be/H ~ 1 - 10 x 10-13, and B/H ~ 1 - 10 x 10-12. It is generally recognized that these isotopes are not of primordial origin, but rather have been produced in the Galaxy, through cosmic-ray nucleosynthesis.

Be and B have been observed in the same pop II stars which show Li and in particular there are a dozen or so stars in which both Be and 7Li have been observed. Thus Be (and B though there is still a paucity of data) can be used as a consistency check on primordial Li. Based on the Be abundance found in these stars, one can conclude that no more than 10-20% of the 7Li is due to cosmic ray nucleosynthesis leaving the remainder (the abundance in Eq. (8) as primordial. This is consistent with the conclusion reached in Ref. [19].

In principle, we can use the abundance information on the other LiBeB isotopes to determine the abundance of the associated GCRN produced 7Li. As it turns out, the boron data is problematic for this purpose, as there is very likely an additional significant source for 11B, namely nu-process nucleosynthesis in supernovae. Using the subset of the data for which Li and Be have been observed in the same stars, one can extract the primordial abundance of 7Li in the context of a given model of GCRN. For example, a specific GCRN model, predicts the ratio of Li/Be as a function of [Fe/H]. Under the (plausible) assumption that all of the observed Be is GCRN produced, the Li/Be ratio would yield the GCRN produced 7Li and could then be subtracted from each star to give a set of primordial 7Li abundances. This was done in [22] where it was found that the plateau was indeed lowered by approximately 0.07 dex. However, it should be noted that this procedure is extremely model dependent. The predicted Li/Be ratio in GCRN models was studied extensively in [23]. It was found that Li/Be can vary between 10 and ~ 300 depending on the details of the cosmic-ray sources and propagation - e.g., source spectra shapes, escape pathlength magnitude and energy dependence, and kinematics.

In contrast, the 7Li / 6Li ratio is much better determined and far less model dependent since both are predominantly produced by alpha - alpha fusion rather than by spallation. The obvious problem however, is the paucity of 6Li data. As more 6Li data becomes available, it should be possible to obtain a better understanding of the relative contribution to 7Li from BBN and GCRN.

The associated BeB elements are clearly of importance in determining the primordial 7Li abundance, since Li is produced together with Be and B in accelerated particle interactions such as cosmic ray spallation. However, these production processes are not yet fully understood. Standard cosmic-ray nucleosynthesis is dominated by interactions originating from accelerated protons and alpha's on CNO in the ISM, and predicts that BeB should be ``secondary'' versus the spallation targets, giving Be propto O-2. However, this simple model was challenged by the observations of BeB abundances in Pop II stars, and particularly the BeB trends versus metallicity. Measurements showed that both Be and B vary roughly linearly with Fe, a so-called ``primary'' scaling. If O and Fe are co-produced (i.e., if O/Fe is constant at low metallicity) then the data clearly contradicts the canonical theory, i.e. BeB production via standard GCR's.

There is growing evidence that the O/Fe ratio is not constant at low metallicity [24], but rather increases towards low metallicity. This trend offers a solution to resolve discrepancy between the observed BeB abundances as a function of metallicity and the predicted secondary trend of GCR spallation [20]. As noted above, standard GCR nucleosynthesis predicts Be propto O2, while observations show Be ~ Fe, roughly; these two trends can be consistent if O/Fe is not constant in Pop II. A combination of standard GCR nucleosynthesis, and nu-process production of 11B may be consistent with current data.

Thus the nature of the production mechanism for BeB (primary vs. secondary) rests with the determination of ratio of O/Fe at low metallicity. In any case, it is clear that given a primary mechanism, it will be dominant in the early phases of the Galaxy, and secondary mechanisms will dominate in the latter stage of galactic evolution. The cross over or break point is uncertain. In Figure 8, a plausible model for the evolution of BeB is shown and compared with the data [25]. Shown by the short dashed lines are standard galactic cosmic-ray nucleosynthesis, which is mostly secondary, but contains some primary production as well. The long dashed curves are purely primary, and in the case of boron, the nu process has been included and this too is primary. The solid curves represent the total Be and B abundance as a function of [O/H]. As one can see such a model fits the data quite well.

Figure 8

Figure 8. Be vs O (top panel) and B vs O (bottom panel). Data shown are found to have a break point as indicated. Models are adjusted to achieve the break point and O/Fe slope of these data.

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