We present Li , Na , Al and Fe abundances of 199 lower red giant branch stars members of the stellar system Omega Centauri , using high-resolution spectra acquired with FLAMES at the Very Large Telescope .
The A ( Li ) distribution is peaked at A ( Li ) \sim 1 dex with a prominent tail toward lower values .
The peak of the distribution well agrees with the lithium abundances measured in lower red giant branch stars in globular clusters and Galactic field stars .
Stars with A ( Li ) \sim 1 dex are found at metallicities lower than [ Fe/H ] \sim –1.3 dex but they disappear at higher metallicities .
On the other hand , Li-poor stars are found at all the metallicities .
The most metal-poor stars exhibit a clear Li-Na anticorrelation , with about 30 % of the sample with A ( Li ) lower than \sim 0.8 dex , while in normal globular clusters these stars represent a small fraction .
Most of the stars with [ Fe/H ] > –1.6 dex are Li-poor and Na-rich .
The Li depletion measured in these stars is not observed in globular clusters with similar metallicities and we demonstrate that it is not caused by the proposed helium enhancements and/or young ages .
Hence , these stars formed from a gas already depleted in lithium .
Finally , we note that Omega Centauri includes all the populations ( Li-normal/Na-normal , Li-normal/Na-rich and Li-poor/Na-rich stars ) observed , to a lesser extent , in mono-metallic GCs .