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Astron. Astrophys. 323, 105-112 (1997)


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Brown dwarfs in the Pleiades cluster

II. J, H and K photometry *

M.R. Zapatero Osorio , E.L. Martín and R. Rebolo

Instituto de Astrofísica de Canarias, Vía Láctea s/n, E-38200 La Laguna, Tenerife, Spain

Received 30 September 1996 / Accepted 16 December 1996

Abstract

We have obtained near-infrared observations of some of the faintest objects so far known towards the Pleiades young stellar cluster, with the purpose of investigating the sequence that connects cluster very low-mass stars with substellar objects. We find that infrared data combined with optical magnitudes are a useful tool to discriminate cluster members from foreground and background late-type field stars contaminating optical surveys. The bottom of the Pleiades sequence is clearly defined 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 substellar limit for HHJ 6, while they are 0.080 and 0.045 [FORMULA] 0.010  [FORMULA] for PPl 15. These masses are consistent with the constraints imposed by the published lithium observations of these Pleiads.

We find a single object infrared sequence in the Pleiades cluster connecting very low-mass stars and brown dwarfs. We propose that the substellar mass limit ([FORMULA] 0.075  [FORMULA]) in the Pleiades ([FORMULA] 120 Myr) takes place at absolute magnitudes [FORMULA]  = 12.4, [FORMULA]  = 10.1, [FORMULA]  = 9.4 and [FORMULA]  = 9.0 (spectral type M7). Cluster members fainter by 0.2 mag in the I -band and 0.1 mag in the K -band should be proper brown dwarfs. The star-brown dwarf frontier in the Hyades cluster (600 Myr) would be located at [FORMULA]  = 15.0, [FORMULA]  = 11.6, [FORMULA]  = 10.8 and [FORMULA]  = 10.4 (spectral type around M9). Fon an age older than 1000 Myr we estimate that brown dwarfs are fainter than [FORMULA]  = 10.9 (spectral type later than M9.5).

Key words: stars: pre-main sequence – stars: late-type – stars: low-mass, brown-dwarfs – open clusters: Pleiades

* Based on observations made with the William Herschel Telescope, operated on the island of La Palma by the Royal Greenwich Observatory in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC), Spain.

Send offprint requests to: M.R. Zapatero Osorio, e-mail: mosorio@iac.es

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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