Cool stars in NGC 2547 and pre-main-sequence lithium depletion

Abstract

We present the results of a spectroscopic survey of X-ray-selected, low-mass candidate members of the young open cluster NGC 2547. Using a combination of photometry, spectroscopic indices and radial velocities, we refineour candidate list and then use our spectroscopy to study the progression of lithium depletion in low-mass pre-main-sequence stars. We derive lithium abundances or upper limits for all our candidate members, which have effective temperatures in the range 5000 > T e f f > 3200 K, and compare these with predictions for lithium burning and depletion provided by a number of models and also with the lithium depletion seen in younger and older stars. We find that some models can reproduce the lithium abundance pattern of NGC 2547 if the cluster has an age of ≃20-35 Myr, which is also indicated by fits to low-mass isochrones in the Hertzsprung-Russell diagram. However, the lack of significant further lithium depletion between NGC 2547 and older clusters argues for an age of at least 50 Myr, more in keeping with the lack of lithium observed in even fainter NGC 2547 candidates. We show that reconciliation of these age estimates may require additions to the physics incorporated in current generations of pre-main-sequence models.