Low-mass members of the young cluster IC 4665 and pre-main-sequence lithium depletion

We have used fibre spectroscopy to establish cluster membership and examine premain-sequence (PMS) lithium depletion for low-mass stars (spectral types F to M) in the sparse young (≃ 30Myr) cluster IC 4665. We present a filtered candidate list of 40 stars that should contain 75 per cent of single cluster members with 11.5 < V < 18 in the central square degree of the cluster. Whilst Fand G-type stars in IC 4665 have depleted little or no lithium, the Kand early M-type stars have depleted more Li than expected when compared with similar stars in other clusters of known age. An empirical age estimate based on Li-depletion among the late-type stars of IC 4665 would suggest it is older than 100Myr. This disagrees entirely with ages determined either from the nuclear turn-off, from isochronal matches to low-mass stars or from the re-appearance of lithium previously found in much lower mass stars (the “lithium depletion boundary”). We suggest that other parameters besides age, perhaps composition or rotation, are very influential in determining the degree of PMS Li-depletion in stars with M > 0.5M⊙. Further work is required to identify and assess the effects of these additional parameters, particularly to probe conditions at the interface between the sub-photospheric convection zone and developing radiative core. Until then, PMS Li depletion in Fto early M-type stars cannot be confidently used as a precise age indicator in young clusters, kinematic groups or individual field stars.