Keck spectra of brown dwarf candidates and a precise determination of the lithium depletion boundary in the α persei open cluster

We have identified 27 candidate very low mass members of the relatively young α Persei open cluster from a 6 deg² CCD imaging survey. Based on their I magnitudes and the nominal age and distance to the cluster, these objects should have masses less than 0.1 script M_⊙ if they are cluster members. We subsequently obtained intermediate-resolution spectra of 17 of these objects using the Keck II telescope and Low-Resolution Imaging Spectrograph (LRIS). We also obtained near-IR photometry for many of the stars. Our primary goal was to determine the location of the "lithium depletion boundary" and, hence, to derive a precise age for the cluster. Most of our program objects have radial velocities consistent with cluster membership, moderately strong Hα emission, and spectral types of M5.5 to M8, as expected from their (R - I)_C colors. We detected lithium with equivalent widths greater than or equal to 0.4 Å in five of the program objects. We have constructed a color-magnitude diagram for the faint end of the α Persei main sequence, including stars for which high signal-to-noise spectra in the region of the lithium λ6708 absorption line have been obtained. These data allow us to accurately determine the α Persei single-star lithium depletion boundary at M(I_C) = 11.47, M_bol = 11.42, (R-I)_CO = 2.12, spectral type M6.0. By reference to theoretical evolutionary models, this converts fairly directly into an age for the a Persei cluster of 90 ± 10 Myr. That age is considerably older than most previously quoted ages for the cluster but consistent with ages estimated from the upper main-sequence turnoff using recent models that include a moderate amount of convective core overshoot. At this age, the two faintest of our spectroscopically confirmed members should be substellar (i.e., brown dwarfs) according to theoretical models.