OPTICAL SPECTROSCOPY OF DENIS MINI-SURVEY BROWN DWARF CANDIDATES

We present optical (6500-9200 Angstrom) spectroscopy of eight cool dwa rfs detected in a 231 square degree ''Mini-survey'' of the Deep NEar I nfrared Survey (DENIS) data. We are able to confirm that the spectral types derived from the Mini-survey infrared spectroscopy are meaningfu l. We provide a spectral sequence which extends beyond the M-dwarf ran ge and into the proposed ''L'' class of dwarfs. The dominant spectral features in the optical for these L-type dwarfs are resonance lines of Cs I and molecular band heads of CrH and FeH. The other dominant feat ure in these L-type spectra is a broad 600 Angstrom absorption dip cen tered on 7700 A, which we identify with extremely strong (equivalent w idth similar to several hundred Angstrom) absorption associated with t he 7664,7698 Angstrom resonance doublet of KI. We find that model atmo spheres which include the effects of molecular condensation without du st opacity (to simulate rapid gravitational settling of dust grains) p roduce significantly better agreement with observed optical spectra fo r L-type dwarfs, than models including dust opacity. This suggests gra vitational settling of dust grains plays an important role in L-dwarf photospheres. The extreme strength of the KI resonance doublet, and di sappearance of TiO and VO, and the consequent dominance of CrH and FeH in L-dwarf spectra offer considerable prospects as sensitive effectiv e temperature diagnostics, even at low spectral resolution.