Spectral classification and effective temperatures of L and T dwarfs based on near-infrared spectra

We have obtained near-infrared spectra of L dwarfs, L/T transition objects, and T dwarfs using the Subaru telescope. The resulting spectra are examined in detail to study their dependence on spectral types. One question is where the methane feature appears: we suggest that it appears at L8 and marginally at L6.5. The water bands at 1.1 and 1.4 mum do not necessarily show steady increase toward later L types but may show inversion at late L types. This does not necessarily imply that the spectral types do not represent a temperature sequence, but rather can be interpreted as the increasing water abundance being offset by the heavier dust extinction in the later L types. We confirm that the FeH 0.99 mum bands appear not only in the late L dwarfs but also in the early T dwarfs. We suggest that FeH could be dredged up by the surface convective zone induced by the steep temperature gradient as a result of the large opacity of the dust cloud itself, and replenished constantly by convection. We have obtained bolometric luminosities of the objects with known parallaxes in our sample, first by integrating the spectra between 0.87 and 2.5 mum, and second by the K-band bolometric correction. Apart from an L3 dwarf, the bolometric luminosities obtained by both methods agree well, and this implies that the K-band bolometric correction, which is obtained using the unified cloudy models, can be applied to obtain the bolometric luminosities and effective temperatures of the L and T dwarfs with known parallaxes from the literature. The relation between the effective temperature and spectral type derived from the K-band bolometric correction shows monotonic behavior throughout the L-T sequence.