Detection of the lithium depletion boundary in the young open cluster IC 4665

Abstract

The so-called lithium depletion boundary (LDB) provides a secure and independent tool for deriving the ages of young open clusters.In this context, our goal is to determine membership for a sample of 147 photometrically selected candidates of the young open cluster IC 4665 and to use confirmed members to establish an age based on the LDB. Employing the FLAMES multi-object spectrograph on VLT/UT2, we have obtained intermediate-resolution spectra of the cluster candidates. The spectra were used to measure radial velocities and to infer the presence of the Li I 670.8 nm doublet and Halpha emission. We have identified 39 bona fide cluster members based on radial velocity, Halpha emission, and Li absorption. The mean radial velocity of IC 4665 is found to be vrad=-15.95 +/- 1.13 km/s. Confirmed cluster members display a sharp transition in magnitude between stars with and without lithium, both in the Im vs. Im-z and in the Ks vs. Im-Ks diagrams.From this boundary, we deduce a cluster age of 27.7^(+4.2)_(-3.5) +/- 1.1 +/- 2 Myr. IC 4665 is the fifth cluster for which an LDB age has been determined, and it is the youngest cluster among these five. Thus, the LDB is established from relatively bright stars still in the contracting pre-main sequence phase. The mass of the boundary is M*=0.24 +/- 0.04 Msun. The LDB age agrees well with the ages derived from isochrone fitting of both low and high mass, turn-off stars, a result similar to what is found in the slightly older NGC 2547.