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

Abstract

We report the discovery of significant depletion of 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 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 depletion region (1) is quite wide, with clear depletion seen from 6000 to 6700 K or hotter, (2) shows a significant dispersion in abundance at all Teff, even with stars of the same Teff, 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 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 depletion properties, the Hyades gap may well be wider than is usually recognized.