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
), 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
10 = 1.5 -
4.5, standard BBN predicts abundances of
the observed abundances found in Pop II
halo stars are: 6Li/H
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
In contrast, the 7Li / 6Li ratio is much better
determined and far less model
dependent since both are predominantly produced by
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
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
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
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.
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.
-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.
-
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.
's on CNO in the ISM,
and predicts that BeB should be ``secondary'' versus the spallation
targets, giving
Be 
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.
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
-process production of 11B
may be consistent with current data.
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.
