The NIRSPEC Brown Dwarf Spectroscopic Survey. I. Low-resolution near-infrared spectra

We present the first results of a near-infrared (0.96-2.31 mum) spectroscopic survey of M, L, and T dwarfs obtained with NIRSPEC on the Keck II telescope. Our new survey has a resolving power of R=lambda/Deltalambdasimilar to2000 and is comprised of two major data sets: 53 J-band (1.14-1.36 mum) spectra covering all spectral types from M6 to T8 with at least two members in each spectral subclass (wherever possible), and 25 flux-calibrated spectra from 1.14 to 2.31 mum for most spectral classes between M6 and T8. Sixteen of these 25 objects have additional spectral coverage from 0.96 to 1.14 mum to provide overlap with optical spectra. Spectral flux ratio indexes for prominent molecular bands are derived, and equivalent widths (EWs) for several atomic lines are measured. We find that a combination of four H2O and two CH4 band strengths can be used for spectral classification of all these sources in the near-infrared and that the H2O indexes are almost linear with spectral type from M6 to T8. The H2O indexes near 1.79 and 1.96 mum should remain useful beyond T8. In the near-infrared a notable feature at the boundary between the M and L types is the disappearance of relatively weak (EWsimilar to1-2 Angstrom) atomic lines of Al I and Ca I, followed by Fe I around L2. At the boundary between L and T dwarfs it is the appearance of CH4 in all near-infrared bands (J, H, and K) that provides a significant spectral change, although we find evidence of CH4 as early as L7 in the K band. The FeH strength and the equivalent width of the K I lines are not monotonic, but in combination with other factors provide useful constraints on spectral type. The K I lines are sensitive to surface gravity. The CO band strength near 2.30 mum is relatively insensitive to spectral class. The peak calibrated flux (F-lambda) in the 0.96-2.31 mum region occurs near 1.10 mum at M6 but shifts to about 1.27 mum at T8. In addition, the relative peak flux in the J, H, and K bands is always in the sense J>H>K except around L6, where the differences are small. One object, 2MASS 2244+20 (L6.5), shows normal spectral behavior in the optical but has an infrared spectrum in which the peak flux in J band is less than at H and K.