The lithium problem

The theory that predicts how the lightest elements formed after the Big Bang has hitherto failed to explain the amount of cosmic lithium. The detection of interstellar lithium beyond the Milky Way gives this theory a boost. See Article p.121

Our knowledge of the abundances of light elements, such as hydrogen, helium and lithium, in the early Universe has relied on measurements of the chemical content of the atmospheres of old stars in the Milky Way's halo. These observations have long puzzled astronomers because they are in partial disagreement with theoretical predictions, which are based on the Big Bang nucleosynthesis theory and on a precise determination of the cosmic ratio of baryons (particles such as protons and neutrons) to photons. The measured 'primordial' amounts of hydrogen and helium match the predictions, but that of lithium does not. Elsewhere in this issue, Howk and colleagues1 (page 121) report a measurement of the abundance of the lithium-7 isotope in the interstellar medium of the Small Magellanic Cloud, a dwarf galaxy neighbouring the Milky Way, that is in accord with the Big Bang nucleosynthesis theory.

The nuclei of hydrogen, helium and lithium were created when the Universe was between 2 and 5 minutes old, after the hot primordial plasma had cooled sufficiently for protons and neutrons to form2. However, the abundance of lithium is billions of times lower than that of hydrogen and helium. This is because lithium is more prone to being destroyed in stars than hydrogen and helium are, and there are not many processes by which lithium is produced.

Astronomers have long thought that the primordial abundance of lithium is preserved in our Galaxy's stars that are especially old and comparatively cool. Stars have a layered structure. Nuclear-fusion reactions take place in the stars' inner (and hotter) regions but not in their outermost layers. Therefore, the composition of the outermost layers should indicate the chemical content of the matter from which a star has formed. For very old stars, such surface chemical abundances should be close to the primordial values. For younger stars, which formed from material that contained the nuclear-fusion products of previous generations of stars, the surface abundances should be different.

To test the theoretical predictions of the Big Bang nucleosynthesis (BBN) theory, we need to identify astronomical objects in which the primordial abundance values are preserved as much as possible, and we need to account for any remaining influences of chemical evolution. In the early 1980s, astronomers discovered3 that old, dwarf stars in our Galaxy — Sun-like stars that are poor in metals (elements other than hydrogen and helium) — share the same lithium abundance irrespective of their temperature and metal content. This 'plateau' was readily interpreted as evidence that the constant lithium abundance was primordial.

Using observations of the cosmic microwave background4 (relic radiation from the Big Bang) obtained by the Wilkinson Microwave Anisotropy Probe satellite, researchers have been able to make an accurate measurement of the cosmic ratio of baryons to photons. Combined with this measurement, the BBN theory predicts an abundance of the lithium-7 isotope that is about four times that inferred from measurements of old, metal-poor stars in the Milky Way's halo5. This mismatch constitutes the 'lithium problem'. The solution to this problem can be sought either by considering modifications to the BBN theory, or by identifying processes by which lithium is destroyed in old, metal-poor halo stars so as to cause the primordial lithium abundance to have evolved, over the stars' lifetimes, to the observed plateau.

An alternative route for tackling the lithium problem — and the one adopted by Howk and colleagues in their study — is to determine the lithium abundance of metal-poor interstellar gas. This approach is unaffected by those processes that can alter the chemical content of stellar atmospheres over time. Howk et al. obtained high-quality spectroscopic observations of the lithium spectral line in the metal-poor gas of the Small Magellanic Cloud (Fig. 1). They then derived the total lithium abundance in the galaxy's interstellar medium. This derivation is a difficult task. It requires knowledge of the ionization fraction of lithium and an accurate determination of the amount of lithium locked in interstellar dust grains. The authors used several approaches to account for and measure these quantities. They found that the present-day abundance of interstellar lithium in the Small Magellanic Cloud is almost equal to the BBN predictions.

Figure 1: The Small Magellanic Cloud.


Howk et al.1 find that the amount of interstellar lithium in the Milky Way's neighbouring Small Magellanic Cloud galaxy is in agreement with the predictions of the Big Bang nucleosynthesis theory. The galaxy is seen here in infrared light collected by the Herschel Space Observatory and the Spitzer Space Telescope.

Howk and colleagues' results are therefore good news for BBN theory. But how can the stellar observations be brought into agreement with the theory? There are many mechanisms that can destroy lithium in stars, all of which imply that the material is processed at temperatures exceeding 2.5 million kelvin. It is, however, difficult to argue that the same mechanism can account for all of the stars that are depleted of lithium. The latest observations5 of lithium in metal-poor stars in the Galactic halo show a 'meltdown' of the lithium plateau for low metal abundances, such that lithium depletion increases with reduced metal abundance. However, some stars do not follow this trend, and remain on the plateau. This implies that the physics of lithium depletion in metal-poor Sun-like stars is not properly understood.

Magnetic activity, or the presence of a companion star or a giant exoplanet6, can modify the surface abundance of lithium in Sun-like stars. However, it remains to be investigated whether these factors can explain the lithium content of metal-poor Galactic-halo stars. There are several unanswered questions, but Howk et al. provide the first convincing evidence that the lithium abundance in Galactic metal-poor stars is not primordial.


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Correspondence to Garik Israelian.

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Israelian, G. The lithium problem. Nature 489, 37–38 (2012).

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