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Astron. Astrophys. 364, 665-673 (2000)

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5. Conclusions

We have presented here mass measurements for 16 VLMS, with accuracies which range between 0.5 and 5%, and for masses down to below 0.1[FORMULA]. We will shortly publish a few additional masses whose derivation involve long baseline interferometric measurements (Ségransan et al., in prep.). These results only represent a snapshot of the progress of our mass measurement program: we continue to monitor the VLMS binaries with high accuracy radial-velocity observations and adaptive optics imaging, and have started using long baseline interferometry. Most of the masses which we present here will be improved in the future, and additional ones with similar accuracies will become available.

In a companion paper (Delfosse et al. 2000a), we show that these masses provide an impressive validation of the theoretical infrared M-L relations of Baraffe et al. (1998), but point towards low level ([FORMULA]0.5 mag) deficiencies of these models in the V band.

A logical next step for this type of work is the determination of accurate masses for even fainter objects, the very late M dwarfs and the L dwarfs. There is at present a large effort in determining the mass function across the stellar/substellar boundary in young open clusters (Bouvier et al. 1998, Zapatero Osorio et al., in prep.) and in the field (Reid et al. 1999, Delfosse & Forveille 2000b). These programs need an accurate calibration of the mass as a function of both luminosity and age (due to the dominant effect of cooling for brown dwarfs). Up to now such relations are only available from models, which unfortunately meet with new difficulties for temperatures lower than were relevant in this paper, as dust condenses in the atmospheres of very cool dwarfs.

Until very recently, no binary of such mass was known with a period short enough for a mass determination over any realistic time scale. A few are now known (Martín et al. 1999, 2000, Koerner et al. 1999), even though their periods are either shorter or longer than would be ideal for a quick and accurate mass measurement. They will eventually provide mass determinations for brown dwarfs, but additional efforts to find more brown dwarfs binaries, and better suited ones, are certainly more than warranted. Several groups are doing this, following up the late-M and L dwarfs discovered by the SLOAN, 2MASS and DENIS surveys.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001