  By comparing near-infrared spectra with atmosphere models, we infer the
effective temperature, surface gravity, projected rotational velocity, and
radial velocity for 21 very-low-mass stars and brown dwarfs. The unique sample
consists of two sequences in spectral type from M6-M9, one of 5-10 Myr objects
and one of >1 Gyr field objects. A third sequence is comprised of only ~M6
objects with ages ranging from <1 Myr to >1 Gyr. Spectra were obtained in the J
band at medium (R~2,000) and high (R~20,000) resolutions with NIRSPEC on the
Keck II telescope. Synthetic spectra were generated from atmospheric structures
calculated with the PHOENIX model atmosphere code. Using multi-dimensional
least-squares fitting and Monte Carlo routines we determine the best-fit model
parameters for each observed spectrum and note which spectral regions provide
consistent results. We identify successes in the reproduction of observed
features by atmospheric models, including pressure-broadened KI lines, and
investigate deficiencies in the models, particularly missing FeH opacity, that
will need to be addressed in order to extend our analysis to cooler objects.
The precision that can be obtained for each parameter using medium- and high-
resolution near-infrared spectra is estimated and the implications for future
studies of very low mass stars and brown dwarfs are discussed.
