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 10^-13.9 [149], [150], using the 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 - fusion reactions, rather than in spallation (e.g. O + p -> ⁶Li) which is favored by [142] and [150]. The production ratio is ⁶Li / -⁷Li 2 for the - 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, 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.