A Lithium Age for the Young Cluster IC2391

Abstract

We have identified a large number of possible very low mass members of the cluster IC2391 based primarily on their location in an I versus (R-I)_C CM diagram. We have obtained new photometry and low resolution ($Δλ= 2.7$ Å) spectroscopy of 19 of these objects (14.9 $\le$ I_C $\le$ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on their spectral types, radial velocity, EW(NaI8200Å), and H(alpha) emission strengths. One of these stars has a definite lithium detection and two other (fainter) stars have possible lithium detections. We find the lithium depletion boundary in IC2391 is at I_C=16.2, which implies an age for IC2391 of 53$\pm$5 Myr. While this is considerably older than the age most commonly attributed for this cluster (~35 Myr), the correction factor to the IC2391 age is comparable to those recently derived for the Pleiades and alpha Per clusters and can be explained by new models for high mass stars that incorporate a modest amount of convective core overshooting.