WIYN/Hydra Detection of Lithium Depletion in F Stars of the Young Open Cluster M35 and Implications for the Development of the Lithium Gap

We report the discovery of significant depletion of Li on the surfaces of F dwarf stars in the 150 Myr old open cluster M35, analagous to a feature in the 700 Myr old Hyades that has been referred to as the "Li gap." We have caught the gap in the act of forming: using high-resolution, high signal-to-noise ratio WIYN/Hydra observations, we detect Li in all but a few M35 F stars; the maximum depletion lies at least 0.6-0.8 dex below minimally depleted (or undepleted) stars. The M35 Li depletion region (1) is quite wide, with clear depletion seen from 6000 to 6700 K or hotter, (2) shows a significant dispersion in Li abundance at all T eff , even with stars of the same T eff , and (3) contains undepleted stars (as well as depleted ones) in the (narrow) classical Hyades gap region, which itself shows no undepleted stars. All of these M35 Li depletion properties support rotationally induced slow mixing as the primary physical mechanism that forms the gap and argue against other proposed mechanisms, particularly diffusion and steady main-sequence mass loss. When viewed in the context of the M35 Li depletion properties, the Hyades Li gap may well be wider than is usually recognized.