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

¹ Instituto de Astrofísica de Canarias, c/ Vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain e-mail: [gbihain;rrl;zvezda;jap;amt]@ll.iac.es;victor.bejar@gtc.iac.es
² Consejo Superior de Investigaciones Científicas, Spain
³ GTC Project, Instituto de Astrofísica de Canarias
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: [calj;mundt]@mpia-hd.mpg.de

Received: 1 March 2006
Accepted: 3 August 2006

Abstract

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 . The detected sources were correlated with those of previously available optical I-band images (). Using a J versus colour–magnitude diagram, we identified 18 faint red L-type candidates, with magnitudes and colours . If Pleiades members, their masses would span ~. We performed follow-up HK_s-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 1 . Five of the seven other IJ candidates are discarded because their colours are bluer than those of confirmed members. Our least massive proper motion members are BRB 28 and 29 (~25 M_Jup). The J versus sequence of the L-type candidates at 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 and diagrams, suggests that Pleiades and field L dwarfs may have similar spectral energy distributions and luminosities, and thus possibly similar radii. Also, we find for a power-law approximation of the survey mass spectrum in the mass range . 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.