Lithium-depleted stars in the young σ Orionis cluster

Context. Knowledge of the age distribution of stars in young clusters and associations is crucial to constrain models of star formation. HR diagrams of different young clusters and associations suggest the presence of age spreads, but the influence of errors on the derived ages is still largely debated. Determination of lithium abundances in low-mass stars represents an alternative and robust way to infer stellar ages. Aims. We measured lithium in a sample of low mass members of the young (4-5 Myr) sigma Ori cluster with the main goal of investigating its star formation history. Methods. Using the FLAMES multi-object spectrograph on VLT/UT2, we obtained spectra of 98 candidate cluster members. The spectra were used to determine radial velocities, to infer the presence of Ha emission, and to measure the strength of the Li I 670.8 nm absorption line. Results. Using radial velocities, Ha and Li, together with information on X-ray emission, we identified 59 high probability cluster members. Three of them show severe Li depletion. The nuclear ages inferred for these highly depleted stars exceed 10-15 Myr; for two of them these values are in good agreement with the isochronal age, while for the third star the nuclear age exceeds the isochronal one.