Keck Spectra of Brown Dwarf Candidates and a Precise Determination of the Lithium Depletion Boundary in the α Persei Open Cluster

We have identified twenty-seven candidate very low mass members of therelatively young Alpha Persei open cluster from a six square degree CCD imagingsurvey. Based on their I magnitudes and the nominal age and distance to thecluster, these objects should have masses less than 0.1 Msunif they are clustermembers. We have subsequently obtained intermediate resolution spectra ofseventeen of these objects using the Keck II telescope and LRIS spectrograph.We have also obtained near-IR photometry for many of the stars. Our primarygoal was to determine the location of the "lithium depletion boundary" andhence to derive a precise age for the cluster. We detect lithium withequivalent widths greater than or equal to 0.4 \AA in five of the programobjects. We have constructed a color-magnitude diagram for the faint end of theAlpha Persei main sequence. These data allow us to accurately determine theAlpha Persei single-star lithium depletion boundary at M(I$_C$) = 11.47, M(Bol)= 11.42, (R-I)$_{C0}$ = 2.12, spectral type M6.0. By reference to theoreticalevolutionary models, this converts fairly directly into an age for the AlphaPersei cluster of 90 $\pm$ 10 Myr. At this age, the two faintest of ourspectroscopically confirmed members should be sub-stellar (i.e., brown dwarfs)according to theoretical models.Comment: Accepted Ap