CONSTRAINTS TO THE MASSES OF BROWN DWARF CANDIDATES FROM THE LITHIUM TEST

We present intermediate dispersion (0.7-2.2 Angstrom pixel(-1)) optica l spectroscopic observations aimed at applying the ''lithium test'' to a sample of 10 brown dwarf candidates located in the general field, t wo in young open clusters, and two in close binaries. We find evidence for strong Li depletion in all of them, and thus infer lower mass lim its of 0.065 M., depending only slightly (+/- 0.005 M.) on the interio r models. None of the field brown dwarf candidates in our sample appea rs to be a very young (age < 10(8) yr) substellar object. For one of t he faintest proper motion Pleiades members known (V = 20.7) the Li tes t implies a mass greater than similar to 0.08 M., and therefore it is not a brown dwarf. From our spectra we estimate spectral types for som e objects and present measurements of H alpha emission strengths and r adial velocities. Finally, we compare the positions in the H-R diagram of our sample of brown dwarf candidates with the theoretical region w here Li is expected to be preserved (substellar lithium region). We fi nd that certain combinations of temperature calibrations and evolution ary tracks are consistent with the constraints imposed by the observed Li depletion in brown dwarf candidates, while others are not.