4.3. Lithium-7
A similar scenario may be sketched for 7Li. As a weakly bound nuclide, it is easily destroyed when cycled through stars except if it can be kept in the cooler, outer layers. The high lithium abundances observed in the few "super-lithium-rich red giants" provide direct evidence that at least some stars can synthesize post-BBN lithium and bring it to the surface. But, an unsolved issue is how much of this newly-synthesized lithium is actually returned to the ISM rather than mixed back into the interior and destroyed.
With these caveats in mind, in
Figure 4 lithium abundances
are shown as a function of metallicity from a compilation by V. V. Smith
(private communication). Since the quest for nearly primordial lithium is
restricted to the oldest, most metal-poor stars in the Galaxy, stars that
have had the most time to redistribute - and destroy or dilute - their
surface lithium abundances, it is unclear whether the "plateau" at low
metallicities is representative of the primordial abundance of lithium.
Although it seems clear that the lithium abundance in the Galaxy has
increased since BBN, a quantitatively reliable estimate of its primordial
abundance eludes us at present. Given this state of affairs, the most
fruitful approach is to learn about stellar structure and evolution by
comparing the BBN-predicted lithium abundance to those abundances inferred
from observations of the oldest stars, rather than to attempt to use
the stellar observations to constrain the BBN-inferred baryon density.
Concentrating on the low-metallicity, nearly primordial data, it seems
that [Li] 
12 + log(Li/H)
2.2 ± 0.1. This
estimate will be compared to the BBN-predicted lithium abundance using D
as a baryometer and, to the BBN-predicted lithium abundance using the
CBR-inferred
baryon density. Any tension between these BBN-predicted abundances and
that inferred from the Galactic data may provide hints of nonstandard
stellar astrophysics.
 |
Figure 4. Lithium abundances, log
|
(Li)