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8.2. 6Li

The primordial 6Li abundance has not been observed, but 6Li / H has been measured in two stars on the Spite plateau. The abundance is well below that expected from SBBN, but 7Li is used to help determine the primordial BBN 6Li abundance in two ways. First, the presence of 6Li limits the amount of destruction of 7Li , because 6Li is more fragile than 7Li . Second, if the observed 6Li was made prior to the formation of the stars, then some, perhaps much 7Li [139], may have been made by similar processes. The first point is often presented as evidence that the 7Li on the Spite plateau is close to primordial (e.g. less that a factor of two depletion, according to [78]), but the second point is cause for caution.

6Li has been detected in only two stars on the Spite plateau, because the absorption line at 6707.97 Å is weak and fully blended with 7Li at 6707.81 Å . This is a difficult observation. The 6Li makes the absorption line slightly asymmetric, and this is detected using models of the line broadening, which are tested on other absorption lines which are expected to have similar profiles because they arise in the same layers of the stellar atmosphere. Following the impressive first detection by [146] and [147], and Cayrel et al. [148] report 6Li /7Li = 0.052 ± 0.019 in HD84937, while Smith, Lambert & Nissen [139] report 6Li /7Li = 0.06 ± 0.03 in BD+26 3578. It is not known whether these detections are representative of halo stars on the Spite plateau. Most assume that they are, but they could be above normal, perhaps by a lot; Smith et al. report 6Li /7Li = 0.00 ± 0.03 for six other stars.

The SBBN makes 6Li / H appeq 10-13.9 [149], [150], using the eta from D/H, which is 500 times less than the measured abundance of 7 x 10-12 in the two halo stars. The SBBN isotope ratio is 6Li / 7Li = 3 x 10-5 , a factor of 2000 less than observed in these two stars. This is not considered a contradiction with SBBN, because 6Li , and some 7Li at the same time, can be made elsewhere.

The 6Li is usually assumed to have been present in the gas when the stars formed, but it could be made later, e.g. when cosmic rays strike the star or in stellar flares [141]. Production by cosmic rays in the ISM prior to the star formation is most favored [142]. With this assumption, the effects on 7Li can be calculated in two steps. First, determine the ratio of 6Li / 7Li in the production process (the production ratio). Second, correct for the depletion of 6Li in the stars to determine the initial abundance of 6Li . The amount of 7Li produced along with the initial 6Li is then specified.

Cosmic rays in the early ISM could have made 6Li and some 7Li prior to the formation of the Spite plateau halo stars. The production ratio depends on the reaction and energies (e.g. [139]). Two reactions of cosmic rays in the ISM are considered to produce 6Li . Smith, Lambert & Nissen [139] find that 6Li /Be ratios imply that most 6Li was made in alpha - alpha fusion reactions, rather than in spallation (e.g. O + p -> 6Li) which is favored by [142] and [150]. The production ratio is 6Li / -7Li appeq 2 for the alpha - alpha reaction.

Standard stellar models [141] predict that much of the initial 6Li will have been destroyed in the stars. The more that was destroyed, the more 6Li and non-BBN 7Li should have been in the initial gas to give the observed abundances. Depending on the destruction mechanism, the destruction of 6Li may also destroy 7Li , but this is usually ignored.

When we choose the amount of depletion of 6Li , we fix the amount present when the stars formed. If the 6Li has been depleted by a large factor, appeq 100, then the stars would have begun with 6Li /7Li similar to the production ratio, and essentially all of the 7Li would be non-primordial [139], which is an unusual conclusion!

Ryan, Norris and Beers [113] assume that 50% of the 6Li and none of the 7Li was destroyed, and use a production ratio of 1.5 to conclude that the BBN 7Li was 0.84 of that now in the stars. Since nearly all observations of Li are made at low resolution, the 6Li and 7Li lines are not resolved, they correct for the 6Li . If the two stars with observed 6Li are normal, then the BBN 7Li is about 79% of the observed Li absorption.

Many other papers discuss this  topic. Olive & Fields [14] give a summary. Cayrel et al. [148] use models for the formation of Li, Be and B and calculate the expected abundance of 6Li when the star formed, and find that the observed abundance implies little depletion of 6Li , and a 7Li depletion of less than 25%. Vangiono-Flam et al. [150] also argue that 6Li is not much depleted, and find that its BBN abundance, extrapolated back to before the production by spallation, is compatible with a BBN abundance of 3 x 10-13 - 5.6 x 10-14.

All eagerly await the measurement of 6Li , together with Beryllium, in more stars.

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