Four Nearby L Dwarfs

Abstract

We present spectroscopic, photometric and astrometric observations of four\nbright L dwarfs identified in the course of the 2MASS near-infrared survey. Our\nspectroscopic data extend to wavelengths shortward of 5000\\AA in the L0 dwarf\n2MASSJ0746+2000 and the L4 dwarf 2MASSJ0036+1840, allowing the identification\nof absorption bands due to MgH and CaOH. The atomic resonance lines Ca I\n4227\\AA and Na I 5890/5896\\AA are extremely strong, with the latter having an\nequivalent width of 240\\AA in the L4 dwarf. By spectral type L5, the D lines\nextend over $\\sim1000$\\AA and absorb a substantial fraction of the flux emitted\nin the V band, with a corresponding effect on the (V-I) broadband colour. The\nKI resonance doublet at 7665/7699\\AA increases in equivalent width from\nspectral type M3 to M7, but decreases in strength from M7 to L0 before\nbroadening substantially at later types. These variations are likely driven by\ndust formation in these cool atmospheres.\n