Spectroscopy of very low mass stars and brown dwarfs in IC 2391:: Lithium depletion and Hα emission

We have obtained intermediate-resolution optical spectroscopy of 44 candidate very low mass members of the nearby young open cluster IC 2391. Of these, 26 spectra are totally new, 14 had been analyzed in a previous paper, and another four are in common with that paper. These spectra, taken at the Cerro Tololo 4 m and Magellan I and II telescopes, allow us to confirm 33 of them as likely cluster members, based on their spectral types, the presence of Li, and Halpha emission. Among these new cluster members is CTIO-160 (M7), the first IC 2391 candidate to satisfy all criteria for being a substellar member of the cluster, including detection of the Li 6708 8 doublet. With the enlarged membership, we are able to locate the lithium depletion boundary of the cluster more reliably than in the past. On the basis of comparison of several theoretical models, we derive an age of 50+/-5 Myr for IC 2391. We also estimate new ages for the alpha Per and Pleiades clusters; our ages are 85+/-10 and 130+/-20 Myr, respectively. We derive an estimate of the initial mass function of IC 2391 that extends to below the substellar limit and compare it to those of other well-studied young open clusters. The index of the power-law mass function for IC 2391 is alpha=0.96+/-0.12, valid in the range 0.5-0.072 M-circle dot.