Four nearby L dwarfs

We present spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our spectroscopic data extend to wavelengths shortward of 5000 Angstrom in the L0 dwarf 2MASS J0746 + 2000 and the L4 dwarf 2MASS J0036 + 1840, allowing the identification of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I lambda 4227 and Na I lambda lambda 5890/5896 are extremely strong, with the latter having an equivalent width of 240 Angstrom in the L4 dwarf. By spectral type L5, the D lines extend over similar to 1000 Angstrom and absorb a substantial fraction of the flux emitted in the V band, with a corresponding effect on the (V-I) broadband color. The K I resonance doublet at 7665/7699 Angstrom increases in equivalent width from spectral type M3 to M7, but decreases in strength from M7 to L0 before broadening substantially at later types. These variations are likely driven by dust formation in these cool atmospheres.