We present a 0.6−4.1 µm spectroscopic sequence of M, L, and T dwarfs. The spectra have R ≡ λ/∆λ ≈ 2000 from 0.9 to 2.4 µm and R=2500−200 from 2.9 to 4.1 µm. These new data nearly double the number of L and T dwarfs that have reported L-band spectra. The near-infrared spectra are combined with previously published redoptical spectra to extend the wavelength coverage to ∼0.6 µm. Prominent atomic and molecular absorption features are identified including neutral lines of Al, Fe, Mg, Ca, Ti, Na, and K and 19 new weak CH 4 absorption features in the H-band spectra of midto late-type T dwarfs. In addition, we detect for the first time the 0−0 band of the A 4 Π − X 4 Σ − transition of VO at ∼1.06 µm in the spectra of L dwarfs and the P and R branches of the ν 3 band of CH 4 in the spectrum of a T dwarf. The equivalent widths of the refractory atomic features all decrease with increasing spectral type and are absent by a spectral type of ∼L0, except for the 1.189 µm Fe I line which persists to

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We present the design, construction, and performance of SpeX, a medium-resolution 0.8-5.5 mm cryogenic spectrograph and imager, now in operation at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea. The design uses prism cross-dispersers and gratings to provide resolving powers up to R ∼ 2000 simultaneously across 0.8-2.4, 1.9-4.2, or 2.4-5.5 mm, with a 15Љ long slit. A high-throughput low-resolution prism mode is also provided for faint-object and occultation spectroscopy. Single-order 60Љ long-slit R ∼ 200 modes with resolving powers up to are available for extended objects. The spectrograph employs an R ∼ 2000 Aladdin 3 InSb array and uses narrow slits and a spatial scale of 0Љ .15 pixel for optimum sensitivity Ϫ1 1024 # 1024 on point sources. An autonomous infrared slit viewer is used for object acquisition, infrared guiding, and scientific imaging in the wavelength range 0.8-5.5 mm. The imager employs an Aladdin 2 InSb array that 512 # 512 covers a field of view at 0Љ .12 pixel . SpeX was successfully commissioned on IRTF during 2000 Ϫ1 60 # 60 May, June, and July. Astronomical observations are presented to illustrate performance.

We present an analysis of the 0.95Y14.5 m spectral energy distributions of nine field ultracool dwarfs with spectral types ranging from L1 to T4.5. Effective temperatures, gravities, and condensate cloud sedimentation efficiencies are derived by comparing the data to synthetic spectra computed from atmospheric models that self-consistently include the formation of condensate clouds. Overall, the model spectra fit the data well, although the agreement at some wavelengths remains poor due to remaining inadequacies in the models. Derived effective temperatures decrease steadily through the L1YT4.5 spectral types, and we confirm that the effective temperatures of ultracool dwarfs at the L/T transition are nearly constant, decreasing by only $200 K from spectral types L7.5 to T4.5. The condensate cloud properties vary significantly among the L dwarfs in our sample, ranging from very thick clouds to relatively thin clouds with no particular trend with spectral type. The two objects in our sample with very red J À K s colors are, however, best fitted with synthetic spectra that have thick clouds, which hints at a possible correlation between the near-infrared colors of L dwarfs and the condensate cloud properties. The fits to the two T dwarfs in our sample (T2 and T4.5) also suggest that the clouds become thinner in this spectral class, in agreement with previous studies. Restricting the fits to narrower wavelength ranges (i.e., individual photometric bands) almost always yields excellent agreement between the data and models. Limitations in our knowledge of the opacities of key absorbers such as FeH, VO, and CH 4 at certain wavelengths remain obvious, however. The effective temperatures obtained by fitting the narrower wavelength ranges can show a large scatter compared to the values derived by fitting the full spectral energy distributions; deviations are typically $200 K and, in the worst cases, up to 700 K.

We describe an IDL-based package for the reduction of spectral data obtained with SpeX, a medium-resolution, 0.8-5.5 mm cross-dispersed spectrograph and imager for the NASA Infrared Telescope Facility. The package, called Spextool, carries out all the procedures necessary to produce fully reduced spectra including preparation of calibration frames, processing and extraction of spectra from science frames, wavelength calibration of spectra, and flux calibration of spectra. The package incorporates an "optimal extraction" algorithm for pointsource data and also generates realistic error arrays associated with the extracted spectra. Because it is fairly quick and easy to use, requiring minimal user interaction, Spextool can be run by observers at the telescope to estimate the signal-to-noise ratio of their data. We describe the procedures incorporated into Spextool and show examples of extracted spectra.