Pleiades low-mass brown dwarfs: the cluster L dwarf sequence

Aims.We present a search for low-mass brown dwarfs in the Pleiades open cluster. The identification of Pleiades members fainter and cooler than those currently known allows us to constrain evolutionary models for L dwarfs and to extend the study of the cluster mass function to lower masses. Methods.We conducted a 1.8 deg2 near-infrared J-band survey at the 3.5 m Calar Alto Telescope, with completeness $J_{\rm cpl}\sim 19.0$. The detected sources were correlated with those of previously available optical I-band images ($I_{\rm cpl}\sim 22$). Using a J versus $I-J$ colour–magnitude diagram, we identified 18 faint red L-type candidates, with magnitudes $17.4 3.2$. If Pleiades members, their masses would span ~$0.040{-}0.020~M_{\odot}$. We performed follow-up HKs-band imaging to further confirm their cluster membership by photometry and proper motion. Results.Out of 11 IJ candidates with proper motion measurements, we find six cluster members, two non-members and three whose membership is uncertain and depends on the intrinsic velocity dispersion of Pleiades brown dwarfs. This dispersion (>4 mas yr-1) is at least four times that of cluster stars with masses $\ga$1 $M_{\odot}$. Five of the seven other IJ candidates are discarded because their $J-K_{\rm s}$ colours are bluer than those of confirmed members. Our least massive proper motion members are BRB 28 and 29 (~25 MJup). The J versus $I-J$ sequence of the L-type candidates at $J>18$ is not as red as theoretical models predict; it rather follows the field L-dwarf sequence translated to the cluster distance. This sequence overlapping, also observed in the J versus $J-H$ and $J-K$ diagrams, suggests that Pleiades and field L dwarfs may have similar spectral energy distributions and luminosities, and thus possibly similar radii. Also, we find $\alpha= 0.5\pm0.2$ for a power-law approximation ${\rm d}N/{\rm d}M \propto M^{-\alpha}$ of the survey mass spectrum in the mass range $0.5{-}0.026~M_{\odot}$. This value is similar to that of much younger clusters, indicating no significant differential evaporation of low-mass Pleiades members relative to more massive ones.