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

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Abstract
We measured lithium in a sample of low-mass stars ( 0.1-0.3 M ) of the Orion Nebula Cluster. We nd evidence for signican t 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 HR diagram, reinforcing our early ndings (Palla et al. 2005). For the two other stars, the nuclear age is signican tly 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.