Lithium in Blanco 1: Implications for Stellar Mixing

We obtain lithium abundances for G and K stars in Blanco 1, an open cluster with an age similar to or slightly younger than the Pleiades. We critically examine previous spectroscopic abundance analyses of Blanco 1 and conclude that, while there were flaws in earlier work, it is likely that Blanco 1 is close in overall metallicity to the older Hyades cluster and more metal-rich than the Pleiades. However, we find Blanco 1 has Li abundances and rotation rates similar to the Pleiades, contradicting predictions from standard stellar evolution models, in which convective pre-main-sequence (PMS) Li depletion should increase rapidly with metallicity. If the high metallicity of Blanco 1 is subsequently confirmed, our observations imply (1) that a currently unknown mechanism severely inhibits PMS Li depletion; (2) that additional nonstandard mixing modes, such as those driven by rotation and angular momentum loss, are then responsible for main-sequence Li depletion between the ages of Blanco 1 and the Hyades; and (3) that in clusters younger than the Hyades, metallicity plays only a minor role in determining the amount of Li depletion among G and K stars. These conclusions suggest that Li abundance remains a useful age indicator among young (<700 Myr) stars even when metallicities are unknown. If nonstandard mixing is effective in Population I stars, the primordial Li abundance could be significantly larger than present day Population II Li abundances because of prior Li depletion.