Astron. Astrophys. 336, 490-502 (1998)

4. Conclusion

We have identified seventeen brown dwarf candidates in the Pleiades, with the faintest having a mass of order 0.045 based on current models. Based on predictions of the field star densities, and also based on our previous success rate in confirming photometric brown dwarf candidates in the Pleiades, we estimate that of order 70% of these candidates will indeed turn out to be Pleiades members (and hence that they are likely brown dwarfs of age 120 Myr). We have also obtained photometry of two objects previously confirmed as Pleiades brown dwarfs - Calar 3 and Teide 2-, and of three previously identified brown dwarf candidates - Roque 7 and 16, PIZ 1-. Our photometry agrees within 0.1-0.2 mag with previously published photometry, except for PIZ 1 for which our photometry suggests that it is significantly bluer than other Pleiades brown dwarf candidates of the same I magnitude. This may indicate that PIZ 1 is not a member of the Pleiades, despite its very late spectral type (Cossburn et al 1997). If PIZ 1 is a Pleiades member, and if our R-I color for it is reasonably accurate, then we may have underestimated the number of Pleiades brown dwarfs because the boundary between the VLM field stars and the Pleiades locus is bluer than we had thought. This could also account for the fact that, beyond (R-I)=2.55, we do not find any Pleiades BDC candidate even though our completeness limit lies at (R-I)3.0.

With our current estimate of the number of Pleiades brown dwarfs, we predict that objects below the hydrogen burning mass limit make up only a few per cent of the mass of the Pleiades. This estimate is uncertain due primarily to lack of verification that our identified objects are actually Pleiades members and due to the relatively small fraction of the cluster that we have surveyed. The most certain way to verify that our candidate objects are brown dwarfs (and Pleiades members) would be to obtain red spectra of sufficient signal-to-noise to measure the lithium 6708Å equivalent width (and/or to measure an accurate radial velocity). At present this is only possible with the Keck telescopes due to the faintness of these stars, although within a few years other large telescopes should become available which will be capable of spectroscopy of sufficient sensitivity. In any case, even with large telescopes, useful spectroscopy is unlikely to be possible for objects fainter than I 19.5. The next best way to verify Pleiades membership - and which should be good to at least I 21 - would be to determine accurate proper motions for our candidate objects. Due to the good seeing and small pixel size for the CFHT images, this should become possible in only of order five years since the Pleiades motion is about 0.05 arcsec per year. In the near future, it will be useful to obtain near-IR photometry for all of the candidate objects in order to confirm that their colors are compatible with the predicted effective temperatures for Pleiades brown dwarfs. This is particularly necessary because of the possible saturation of the (R-I)_c color index for our faintest stars. We expect that infrared color indices will not saturate, and so there will be a better separation of the Pleiades brown dwarfs from possible field star contaminants once we have a broader wavelength coverage for our photometry.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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