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Description
The cosmological Li problem is the observed discrepancy between Li abundance (A(Li)) measured in Galactic dwarf, old and metal-poor stars (traditionally assumed to be equal to the initial value A(Li)_0_), and that predicted by standard big bang nucleosynthesis (BBN) calculations (A(Li)_BBN_). Here, we attack the Li problem by considering an alternative diagnostic, namely the surface Li abundance of red giant branch stars that in a colour-magnitude diagram populate the region between the completion of the first dredge-up and the red giant branch bump. We obtained high-resolution spectra with the FLAMES facility at the Very Large Telescope for a sample of red giants in the globular cluster M54, belonging to the Sagittarius dwarf galaxy. We obtain A(Li)=0.93+/-0.11dex, translating - after taking into account the dilution due to the dredge-up - to initial abundances (A(Li)_0_) in the range 2.35-2.29 dex, depending on whether or not atomic diffusion is considered. This is the first measurement of Li in the Sagittarius galaxy and the more distant estimate of A(Li)_0_ in old stars obtained so far. The A(Li)_0_ estimated in M54 is lower by ~0.35dex than A(Li)_BBN_, hence incompatible at a level of ~3{sigma}. Our result shows that this discrepancy is a universal problem concerning both the Milky Way and extragalactic systems. Either modifications of BBN calculations, or a combination of atomic diffusion plus a suitably tuned additional mixing during the main sequence, need to be invoked to solve the discrepancy.
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