Infrared Spectra and Spectral Energy Distributions of Late M and L Dwarfs

We have obtained 1.0-2.5 μm spectra at R ~ 600 of 14 disk dwarfs with spectral types M6-L7. For four of the dwarfs, we have also obtained infrared spectra at R ~ 3000 in several narrow intervals. In addition, we present new L' photometry for four of the dwarfs in the sample, which allows improved determinations of their bolometric luminosities. While obtaining the photometry we resolved the L dwarf Denis-P J0205-1159 into an identical pair of objects separated by 0.″35. The spectra together with the published energy distribution for one other L5 dwarf are compared to synthetic spectra generated by upgraded model atmospheres. Good matches are found for 2200 K ≥ Teff ≥ 1900 K (spectral types around M9-L3), but discrepancies exist at Teff ≥ 2300 K (M8) and Teff ≤ 1800 K (L4-L7). At the higher temperatures the mismatches are due to incompleteness in the water vapor opacity line list. At the lower temperatures the disagreement is probably due to our treatment of dust; we assume a photospheric distribution in equilibrium with the gas phase and neglect any diffusion mechanisms. We derive effective temperatures for the sample from the comparison with synthetic spectra and also by comparing our observed total intrinsic luminosities to structural model calculations (which are mostly independent of the atmosphere but are dependent on the unknown masses and ages of the targets). The two derivations agree to about 200 K except for the faintest object in the sample, where the discrepancy is larger. Agreement with other temperature determinations is also ±~200 K, except for the L7 dwarf.