Spectroscopic classification of late-M and L field dwarfs

We present spectra for 12 new ultracool dwarfs found in the DENIS infrared survey. Seven of them have spectral types at the bottom of the M-class (M8- M9.5), and the other five belong to the cooler "L" class. We also present s pectra for the two new L dwarfs found by the EROS 2 proper-motion survey. W e introduce a scheme for L dwarf classification that is based on an extensi on to cooler spectra of a pseudocontinuum ratio previously defined for M dw arfs. For calibrating the spectral subclasses, we use a temperature scale f or late-M and L dwarfs recently obtained by Basri et al, from synthetic spe ctrum fitting of high-resolution profiles of Cs I and Rb I resonance lines. We define that the subclass range from LO to L6 corresponds to the tempera ture range from 2200 K to 1600 K. Our subclasses L0, L1, and L2 agree with recent findings by Kirkpatrick et al., but then they diverge such that our L6 is equivalent to their L8. We find that late-M and L dwarf subclasses ca n be assigned either in the optical with the PC3 index or in the near-infra red with the H2O H-band index. We discuss the main photospheric features pr esent in L dwarf spectra, in particular in the region 400-650 Mn, which has never been shown before. The TiO bands at 549.7, 559.7, 615.9, and 638.4 n m fade with decreasing temperature, but do not vanish until well inside the L domain (similar to L5). The Na I 589.0, 589.6 nm resonance doublet in ou r latest object (L6) becomes the broadest atomic feature ever seen in any c ool dwarf. We do not detect H-alpha emission in our L dwarfs later than L3. We discuss the ages and masses of our objects using their temperatures and absence or presence of lithium. Finally, we compare two L1 dwarfs with dif ferent gravities (one with lithium and one without it) and discuss differen ces in spectral features.