The lithium content of omega Centauri. New clues to the cosmological Li problem from old stars in external galaxies

International audienceContext. A discrepancy has emerged between the cosmic lithium abundance inferred by the WMAP satellite measurement coupled with the prediction of the standard big-bang nucleosynthesis theory, and the constant Li abundance measured in metal-poor halo dwarf stars (the so-called Spite plateau). Several models are being proposed to explain this discrepancy, involving either new physics, in situ depletion, or the efficient depletion of Li in the pristine Galaxy by a generation of massive first stars. The realm of possibilities may be narrowed considerably by observing stellar populations in different galaxies, which have experienced different evolutionary histories. Aims: The omega Centauri stellar system is commonly considered as the remnant of a dwarf galaxy accreted by the Milky Way. We investigate the lithium content of a conspicuous sample of unevolved stars in this object. Methods: We obtained moderate resolution (R = 17 000) spectra for 91 main-sequence/early sub-giant branch (MS/SGB) omega Cen stars using the FLAMES-GIRAFFE/VLT spectrograph. Lithium abundances were derived by matching the equivalent width of the Li i resonance doublet at 6708 Å to the prediction of synthetic spectra computed with different Li abundances. Synthetic spectra were computed using the SYNTHE code along with ATLAS-9 model atmospheres. The stars effective temperatures are derived by fitting the wings of the H_alpha line with synthetic profiles. Results: We obtain a mean content of A(Li) = 2.19 ± 0.14 dex for omega Centauri MS/SGB stars. This is comparable to what is observed in Galactic halo field stars of similar metallicities and temperatures. Conclusions: The Spite plateau seems to be an ubiquitous feature of old, warm metal-poor stars. It exists also in external galaxies, if we accept the current view about the origin of omega Cen. This implies that the mechanism(s) that causes the ``cosmological lithium problem'' may be the same in the Milky Way and other galaxies