The Lithium-Depletion Boundary and the Age of the Young Open Cluster IC 2391

Abstract

In previous programs, we have identified a large number of possible, very low mass members of the young open cluster IC 2391 based on their location in an I versus (RI) c color-magnitude diagram. We have now obtained new photometry and intermediate-resolution (A ˚) spectra of 19 of these objects (Dl 2.7 14.9 ≤ I ≤ c) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on 17.5 their VRI photometry, spectral types, radial velocity, and Ha emission strengths. Higher signal-to-noise ratio spectra were obtained for eight of these probable cluster members in order to measure the strength of the doublet Li l6708 and thus obtain an estimate of the cluster's age. One of these eight stars has a definite lithium detection, and two other (fainter) stars have possible lithium detections. A color-magnitude diagram for our program objects shows that the lithium-depletion boundary in IC 2391 is at. Using recent theoretical model predictions, I 16.2 c we derive an age for IC 2391 of Myr. While this is considerably older than the age most commonly 53 5 attributed to this cluster (∼35 Myr), this result for IC 2391 is comparable to those recently derived for the Pleiades and a Persei clusters and can be explained by new models for high-mass stars that incorporate a modest amount of convective-core overshooting. Subject headings: open clusters and associations: individual (IC 2391)-stars: low-mass, brown dwarfs