Age spreads in star-forming regions: The lithium test in the orion nebula cluster

We present the initial results of a study of the surface lithium abundance in a sample of low-mass members (M-* similar to 0.4 - 1.0 M-circle dot) of the Orion Nebula Cluster (ONC) that provide an independent clock to estimate stellar ages. We report the discovery of a significant depletion of lithium in four stars with estimated masses of similar to 0.4 M, and ages similar to 10 Myr. Comparison with the predictions of numerical and analytical models shows excellent agreement between the isochronal age and lithium depletion timescale for two objects, the first such case for lithium-poor pre-main-sequence stars. Our results bear on the issue of the real age spread in the ONC and hence on the overall duration of the star formation process, indicating that the stellar population did not come into existence in a single, rapid burst.