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

TL;DR

This study identifies the lithium depletion boundary in Blanco 1 to determine its age, using spectroscopic data and membership criteria, resulting in an age estimate of approximately 132 million years.

Contribution

First detection of the lithium depletion boundary in Blanco 1, providing a precise age measurement consistent with other methods and models.

Findings

Li depletion boundary at I=18.78 and I-Ks=3.05

LDB age of 132 ± 24 Myr

Good agreement with other age estimates when using models with convective core overshoot

Abstract

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-Ks 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 A feature to identify the lithium depletion boundary (LDB) in Blanco 1. The I/I-Ks 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=$18.78 \pm 0.24$ and I-Ks=$3.05 \pm 0.10$. Using current pre-main-sequence evolutionary models we determine an LDB age of $132 \pm 24$ 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.