Identification of the Lithium Depletion Boundary and Age of the Southern Open Cluster Blanco 1

We present results from a spectroscopic study of the very low mass members of the Southern open cluster Blanco 1 using the Gemini-N telescope. We obtained intermediate resolution (R ~ 4400) GMOS spectra for 15 cluster candidate members with I ~ 14-20 mag, and employed a series of membership criteria—proximity to the cluster's sequence in an I/I - K_s color-magnitude diagram (CMD), kinematics agreeing with the cluster systemic motion, magnetic activity as a youth indicator—to classify 10 of these objects as probable cluster members. For these objects, we searched for the presence of the Li I 6708 Å feature to identify the lithium depletion boundary (LDB) in Blanco 1. The I/I - K_s CMD shows a clear mass segregation in the Li distribution along the cluster sequence; namely, all higher mass stars are found to be Li poor, while lower mass stars are found to be Li rich. The division between Li-poor and Li-rich (i.e., the LDB) in Blanco 1 is found at I = 18.78 ± 0.24 and I - K_s = 3.05 ± 0.10. Using current pre-main-sequence evolutionary models, we determine an LDB age of 132 ± 24 Myr. Comparing our derived LDB age to upper-main-sequence isochrone ages for Blanco 1, as well as for other open clusters with identified LDBs, we find good chronometric consistency when using stellar evolution models that incorporate a moderate degree of convective core overshoot.

Based on observations with the Gemini-North Telescope, Program ID: GN-2009B-Q-53.