NICS-TNG low-resolution 0.85-2.45 micron spectra of L dwarfs: A near-infrared spectral classification scheme for faint dwarfs

We present complete near-infrared (0.85-2.45 mum), low-resolution (similar to 100) spectra of a sample of 26 disk L dwarfs with reliable optical spect ral type classification. The observations have been obtained with the Near Infrared Camera and Spectrograph at the Telescopio Nazionale Galileo using a prism-based optical element (the Amici device) that provides a complete s pectrum of the source on the detector. Our observations show that low-resol ution near-infrared spectroscopy can be used to determine the spectral clas sification of L dwarfs in a fast but accurate way. We present a library of spectra that can be used as templates for spectral classification of faint dwarfs. We also discuss a set of near-infrared spectral indices well correl ated with the optical spectral types that can be used to classify accuratel y L dwarfs earlier than L6.