Brown dwarfs in the Pleiades cluster

Abstract

Wehaveobtainednear-infraredobservationsofsome of the faintest objects so far known towards the Pleiades young stellar cluster, with the purpose of investigating the sequence thatconnectsclusterverylow-massstarswithsubstellarobjects. Wendthatinfrareddatacombinedwithopticalmagnitudesare a useful tool to discriminate cluster members from foreground and background late-type eld stars contaminating optical sur- veys. The bottom of the Pleiades sequence is clearly dened by the faint HHJ objects as the very low-mass stars approaching the substellar limit, by the transition object PPl 15, which will barely ignite its hydrogen content, and by the two brown dwarfs Calar 3 and Teide 1. Binarity amongst cluster members could account for the large dispersion observed in the faint end of the infrared colour- magnitude diagrams. Two objects in our sample, namely HHJ 6 and PPl 15, are overluminous compared to other members, suggesting a probable binary nature. We have reproduced the photometric measurements of both of them by combining the magnitudes of cluster very low-mass stars and brown dwarfs and using the most recent theoretical evolutionary tracks. The likely masses of the components are slightly above the substel- lar limit for HHJ 6, while they are 0.080 and 0.0450.010 M for PPl 15. These masses are consistent with the constraints imposed by the published lithium observations of these Pleiads. We nd a single object infrared sequence in the Pleiades cluster connecting very low-mass stars and brown dwarfs. We propose that the substellar mass limit (0.075 M )i n the Pleiades (120 Myr) takes place at absolute magnitudes MI = 12.4, MJ = 10.1, MH = 9.4 and MK = 9.0 (spectral type M7). Cluster members fainter by 0.2 mag in the I-band and 0.1 magintheK-bandshouldbeproperbrowndwarfs.Thestar- brown dwarf frontier in the Hyades cluster (600 Myr) would be located at MI = 15.0, MJ = 11.6, MH = 10.8 and MK = 10.4