A revised age greater than 50 Myr for the young cluster IC 4665

Abstract

IC 4665 is one of only a dozen young open clusters with a 'lithium depletion boundary' (LDB) age. Using an astrometrically and spectroscopically filtered sample of cluster members, we show that both the positions of its low-mass stars in Gaia absolute colour-magnitude diagrams and the lithium depletion seen among its K- and early M-stars are discordant with the reported LDB age of $32^{+4}_{-5}$ Myr. Re-analysis of archival spectra suggests that the LDB of IC 4665 has not been detected and that the published LDB age should be interpreted as a lower limit. Empirical comparisons with similar data sets from other young clusters with better-established LDB ages indicate that IC 4665 is bracketed in age by the clusters IC 2602 and IC 2391 at 55 ± 3 Myr.