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Old Stars in Young Clusters: Lithium-depleted Low-Mass Stars of the Orion Nebula Cluster

We measured lithium in a sample of low-mass stars (~0.1-0.3 M☉) of the Orion Nebula cluster. We find evidence for significant Li depletion in four high-probability members, corresponding to nuclear ages between ~15 and 30 Myr. In two cases, there is excellent agreement between the mass and age based on models of Li burning and those derived from the H-R diagram, reinforcing our early findings. For the two other stars, the nuclear age is significantly larger than the isochronal one. Several Li-depleted stars display accretion activity, veiling, and emission lines. We discuss empirical evidence in favor of the old nuclear age and the implications on the star formation history of the Orion cluster.